Health & Wearable Glossary

A sensor inside wearables that measures changes in velocity along one or more axes, used to detect steps, movement patterns, activity type, and sleep movements.

A non-invasive method of monitoring rest and activity cycles using a wrist-worn accelerometer. Commonly used in sleep research as an alternative to polysomnography for tracking sleep-wake patterns over multiple days.

Calories burned through physical activity and exercise, as opposed to those used for basic metabolic functions like breathing and digestion. Wearables estimate active calories using heart rate, movement data, and user profile information.

A low-intensity exercise session performed on rest days to promote blood flow, reduce muscle soreness, and accelerate the recovery process without adding significant training load.

A Fitbit metric that awards points for time spent in fat-burn, cardio, or peak heart rate zones. Double points are earned for time in higher-intensity zones, with a weekly target of 150 minutes aligned with WHO exercise guidelines.

An AI-driven approach to workout programming that automatically adjusts training plans based on real-time recovery data, performance trends, and individual responses from wearable sensors.

Specialized connective tissue composed mainly of fat cells (adipocytes) that stores energy, insulates the body, and produces hormones like leptin. Excess adipose tissue is associated with metabolic disorders.

The exercise intensity at which the body transitions from primarily fat oxidation to increasing reliance on carbohydrate metabolism, typically around 60-70% of max heart rate.

An irregular and often rapid heart rhythm originating in the atria that increases the risk of stroke, heart failure, and other cardiovascular complications. Several consumer wearables, including Apple Watch and Fitbit, have received FDA clearance to detect signs of AFib.

A step-by-step set of rules or calculations used by software to process data and produce a result. Health wearables rely on proprietary algorithms to convert raw sensor data into metrics like sleep stages, calorie burn, and stress levels.

The physiological process by which the body adapts to reduced oxygen availability at high elevations, involving increased red blood cell production and ventilation rate.

A sensor in wearables that detects surrounding light levels to automatically adjust screen brightness and contribute to light exposure tracking for circadian health.

Active Matrix Organic Light-Emitting Diode, a display technology used in many smartwatches. AMOLED screens offer vibrant colors, deep blacks, and energy efficiency since individual pixels can be turned off completely.

The exercise intensity above which lactate accumulates in the blood faster than it can be cleared, marking a shift to predominantly anaerobic energy production. Training near this threshold improves the ability to sustain high-intensity effort.

The branch of the nervous system that controls involuntary functions including heart rate, digestion, and respiration. HRV measurements from wearables provide insight into ANS balance between the sympathetic and parasympathetic divisions.

A set of protocols that allows different software applications to communicate with each other. Health platforms like Garmin Connect, WHOOP, and Oura provide APIs that let third-party apps access user health data.

A diagnostic metric that counts the number of apnea (complete breathing cessation) and hypopnea (partial breathing reduction) events per hour of sleep. AHI is the primary measure used to classify the severity of obstructive sleep apnea.

Apple's integrated health data platform on iOS that aggregates data from the Apple Watch, iPhone sensors, and third-party apps into a unified health record. HealthKit provides a developer framework for reading and writing health data.

A pressure-based sensor in wearables that measures altitude changes by detecting atmospheric pressure variations, used for tracking floors climbed, elevation gain during hikes, and weather-related pressure changes.

The minimum number of calories your body needs at complete rest to maintain vital functions such as breathing, circulation, and cell production. BMR accounts for roughly 60-75% of total daily energy expenditure.

A body composition measurement technique that sends a small electrical current through the body and estimates fat mass, lean mass, and water content based on tissue resistance. Used in smart scales and some wearables.

A sensor found in advanced wearables and smart scales that measures the resistance of body tissues to a small electrical current, used for body composition estimation and hydration tracking.

An estimate of how old your body appears to be based on biomarkers like HRV, VO2 max, blood pressure, and other health indicators, as opposed to your chronological age.

A measurable biological indicator that reflects a physiological state, disease process, or response to treatment. Wearable-derived biomarkers include resting heart rate, HRV, SpO2, and body temperature trends.

The concentration of sugar (glucose) in the bloodstream, a critical metric for metabolic health and diabetes management. Continuous glucose monitors provide real-time tracking of blood glucose fluctuations.

The percentage of hemoglobin molecules in the blood that are carrying oxygen, normally between 95-100%. Many wearables track SpO2 during sleep to detect potential breathing disturbances.

The force exerted by circulating blood on the walls of blood vessels, expressed as systolic over diastolic pressure in mmHg. Some wearables, including certain Samsung Galaxy Watch models, can estimate blood pressure.

A wireless communication protocol optimized for low power consumption, used by nearly all wearable devices to sync data with smartphones and other devices.

A Garmin proprietary metric that estimates your body's energy reserves on a scale of 0-100, based on HRV, stress, sleep quality, and activity data. It helps users decide when to push hard and when to rest.

The proportion of fat, muscle, bone, and water in the body. Understanding body composition provides a more complete picture of physical health than weight alone, and can be measured using DEXA, bioimpedance, and other methods.

The proportion of total body weight that is composed of fat tissue, used as a key indicator of health and fitness level. Healthy ranges vary by age and sex but are generally 10-20% for men and 18-28% for women.

A ratio of weight to height squared (kg/m2) used as a screening tool for weight categories. While widely used, BMI does not directly measure body fat and can be misleading for muscular individuals.

The internal temperature of the body, typically around 37C (98.6F). Wearables track skin temperature variations to detect illness onset, menstrual cycle phases, and recovery status.

The total amount of fluid in the body expressed as a percentage of body weight. Optimal levels are around 60% for men and 50% for women. Bioimpedance scales estimate body water as part of composition analysis.

The amount of mineral matter per square centimeter of bone, an important indicator of skeletal health and osteoporosis risk. DEXA scans are the gold standard for measuring bone density.

A resting heart rate below 60 beats per minute. While common in highly trained athletes, bradycardia can indicate underlying cardiac issues in sedentary individuals.

The number of steps per minute during running or walking, used to assess running efficiency. Optimal running cadence is generally considered to be around 170-180 spm, though it varies by individual and pace.

The process of adjusting a sensor or measurement system to ensure accuracy. GPS watches may calibrate stride length using satellite data, and blood pressure cuffs require periodic calibration against clinical-grade devices.

A state where calorie intake is less than calorie expenditure, resulting in the body using stored energy (primarily fat) for fuel, leading to weight loss over time.

The volume of blood pumped by the heart per minute, calculated as heart rate multiplied by stroke volume. A key measure of cardiovascular performance that increases with aerobic fitness.

The gradual increase in heart rate during prolonged exercise at a constant intensity, caused by dehydration, rising body temperature, and reduced stroke volume.

A wearable sensor inserted under the skin that continuously measures interstitial glucose levels, providing real-time data and trends. Brands like Dexcom, Abbott, and Ultrahuman offer CGMs for medical and wellness use.

Your biological predisposition toward being a morning person (lark), evening person (owl), or somewhere in between, determined by your internal circadian clock and genetics.

The roughly 24-hour internal biological clock that regulates sleep-wake cycles, hormone release, body temperature, and other physiological processes. Light exposure is the primary external cue that synchronizes circadian rhythm.

A carefully designed research study conducted with human participants to evaluate the safety and effectiveness of medical treatments, devices, or interventions. Wearable companies often conduct clinical trials to support regulatory submissions.

A feature where a wearable device uses the GPS receiver in a paired smartphone rather than having its own built-in GPS. This approach saves cost and battery life but requires carrying the phone during outdoor activities.

The temperature of the internal organs and blood, maintained within a narrow range (36.5-37.5C). Some advanced wearables estimate core temperature from skin temperature readings and proprietary algorithms.

A steroid hormone produced by the adrenal glands in response to stress. It follows a circadian rhythm, peaking in the morning to promote wakefulness. Chronically elevated cortisol disrupts sleep and impairs recovery.

The protection and governance of personal health information collected by wearables and apps, regulated by frameworks like HIPAA, GDPR, and individual platform policies.

The most restorative stage of non-REM sleep (stage N3), characterized by slow delta brain waves. Critical for physical recovery, immune function, and memory consolidation. Adults typically need 1-2 hours per night.

A condition where the body loses more fluids than it takes in, impairing physical performance, cognitive function, and recovery. Some wearables infer hydration status through changes in HRV, skin temperature, and bioimpedance.

Dual-Energy X-ray Absorptiometry, a medical imaging technique considered the gold standard for measuring body composition. DEXA scans provide detailed breakdowns of fat mass, lean mass, and bone mineral density across body regions.

A quantifiable physiological or behavioral measure collected through digital devices that can serve as an indicator of health status, disease progression, or treatment response.

A broad category encompassing the use of digital technologies including wearables, telemedicine, AI, and mobile health apps to improve health outcomes. The sector has grown rapidly alongside advances in sensors and connectivity.

A GPS system that receives signals on two different frequency bands (typically L1 and L5), resulting in significantly improved positional accuracy. High-end sports watches from Garmin, COROS, and Apple use dual-band GPS.

A recording of the electrical activity of the heart over time. Consumer wearables like Apple Watch, Samsung Galaxy Watch, and Withings ScanWatch offer single-lead ECG that can detect irregular rhythms including atrial fibrillation.

A measure of electrical changes in skin conductance caused by sweat gland activity, driven by the sympathetic nervous system. Fitbit Sense and some other wearables use EDA sensors to assess stress responses.

Processing data locally on or near the wearable device rather than sending all data to the cloud. On-device edge computing enables real-time health alerts, reduces latency, and enhances data privacy.

Minerals like sodium, potassium, magnesium, and calcium that carry electrical charges and are essential for hydration, nerve function, and muscle contraction. Maintaining electrolyte balance is critical during prolonged exercise.

Garmin's proprietary optical heart rate sensor technology, found on the underside of many Garmin watches. The latest generation (Elevate v4) uses multiple LEDs and photodiodes for improved accuracy during exercise.

The increased rate of oxygen intake following exercise, representing the energy required to restore the body to its pre-exercise state. Used by some wearables as a proxy for training load and caloric afterburn.

A fixed time interval used for data collection and analysis in health monitoring. In actigraphy and sleep science, data is recorded in epochs of 30 or 60 seconds, with algorithms classifying each epoch as sleep or wake.

The total weight of all adipose (fat) tissue in the body, including essential fat needed for organ protection and stored fat used as energy reserves. Tracked through body composition analysis.

The rate at which the body breaks down fat for energy during exercise, typically highest at moderate intensities and declining as exercise intensity increases.

An authorization from the U.S. Food and Drug Administration that a medical device is substantially equivalent to an already-marketed device. Many wearable health features, such as ECG and AFib detection, require FDA 510(k) clearance.

A standard for exchanging healthcare information electronically, developed by HL7 International. FHIR enables wearable health data to integrate with electronic health records and clinical systems.

The embedded software on a wearable device that controls its hardware functions, sensor algorithms, and user interface. Firmware updates can improve accuracy, add features, and fix bugs without changing hardware.

A paid subscription service from Fitbit that provides advanced health insights, detailed sleep analysis, guided workouts, mindfulness sessions, and wellness reports beyond the free tier of the Fitbit app.

An estimated biological age based on cardiovascular fitness metrics like VO2 max, often showing a younger fitness age for people with above-average cardiorespiratory fitness. Offered by Garmin and other platforms.

A wearable electronic device that monitors and records fitness-related metrics such as steps, distance, calories burned, heart rate, and sleep. Modern fitness trackers range from simple step counters to full-featured health platforms.

The highest average power output a cyclist can sustain for approximately one hour, measured in watts. FTP is a cornerstone metric for structured cycling training and is used to define personalized power-based training zones.

A measure of the electrical conductivity of the skin, which varies with moisture level (sweat). Used to detect stress, emotional arousal, and sympathetic nervous system activity in some health wearables.

Garmin's companion app and web platform for viewing, analyzing, and sharing health and fitness data. It offers training plans, challenges, insights, and integration with third-party services like Strava and MyFitnessPal.

A physiological analytics engine acquired by Garmin that powers many of its advanced health and performance metrics, including VO2 max estimation, Training Load, Body Battery, and stress tracking.

A simple sugar that serves as the primary source of energy for the body's cells. Blood glucose levels are influenced by diet, exercise, stress, and hormones, and are monitored by diabetics and health-conscious individuals alike.

The degree of fluctuation in blood glucose levels throughout the day. Lower variability is associated with better metabolic health and reduced disease risk. CGM data makes this metric accessible to consumers.

A ranking system for carbohydrate-containing foods based on how quickly they raise blood glucose levels, on a scale of 0-100. Low-GI foods produce a gradual rise in blood sugar.

The stored form of glucose in muscles and the liver, serving as the primary fuel source during high-intensity exercise and fasting states. Glycogen depletion contributes to fatigue during prolonged exercise.

The umbrella term for satellite-based navigation systems including GPS (US), GLONASS (Russia), Galileo (EU), and BeiDou (China). Multi-GNSS support in wearables improves accuracy by using signals from multiple constellations.

Google's unified health data platform for Android that allows apps and devices to share health and fitness data with user permission, serving as a central hub for health information on Android.

The most common light wavelength used in wrist-based optical heart rate sensors. Green light is strongly absorbed by blood, making it effective for detecting pulse-related changes in blood volume beneath the skin.

The duration each foot spends on the ground during a running stride, measured in milliseconds. Shorter ground contact time generally indicates better running efficiency. Tracked by Garmin, Polar, and other devices.

A motion sensor in wearables that measures rotational movement and orientation, working alongside the accelerometer for more accurate activity recognition, swim stroke detection, and gesture control.

Vibration-based alerts used by wearables to notify users of health events, heart rate zone changes, incoming messages, navigation cues, or to provide guided breathing prompts.

A composite metric generated by health platforms that combines multiple biomarkers into a single number representing overall health status and trends over time.

The number of years a person lives in good health, free from serious chronic disease and disability. Healthspan optimization is a growing focus in the longevity and preventive health communities.

The number of times the heart beats per minute (bpm). Resting heart rate is a key indicator of cardiovascular fitness, with lower values generally indicating better aerobic conditioning.

The rate at which heart rate decreases after stopping exercise. A greater drop in the first minute (typically 15-25 bpm or more) indicates better cardiovascular fitness and autonomic function.

The difference between maximum heart rate and resting heart rate. HRR is used in the Karvonen Formula to calculate personalized heart rate training zones that account for individual fitness levels.

The variation in time intervals between consecutive heartbeats, measured in milliseconds. Higher HRV generally indicates better cardiovascular fitness and recovery. It is a key metric used by Oura, WHOOP, and Garmin.

Five intensity ranges based on percentage of maximum heart rate, used to structure training for different goals: fat burning (Zone 1-2), endurance (Zone 3), threshold (Zone 4), and VO2 max (Zone 5).

The iron-containing protein in red blood cells that transports oxygen from the lungs to body tissues. Its oxygen-carrying capacity is what SpO2 sensors measure in wearables.

A training method alternating short bursts of maximum-effort exercise with recovery periods, proven to improve cardiovascular fitness and metabolic rate efficiently in shorter workout durations.

The monitoring of fluid intake and body water levels through manual logging or wearable sensors like bioimpedance to maintain optimal hydration status during daily life and exercise.

A body composition assessment method that measures body density by comparing weight on land to weight while fully submerged in water. Considered highly accurate but less accessible than other methods.

Chronically elevated blood pressure (typically above 130/80 mmHg), a major risk factor for heart disease, stroke, and kidney disease. Some wearables now offer blood pressure estimation features.

A graphical representation of sleep stages throughout the night, showing transitions between wake, light sleep, deep sleep, and REM stages over time. Most sleep-tracking wearables display a hypnogram in their app.

The total opposition to the flow of an alternating electrical current through the body. In body composition analysis, impedance values at different frequencies help distinguish between fat mass, lean mass, and water content.

A light-emitting diode that emits infrared light, used in wearables primarily for blood oxygen (SpO2) measurements. Infrared light penetrates deeper into tissue and is differentially absorbed by oxygenated and deoxygenated hemoglobin.

Garmin's satellite communication technology that enables two-way messaging and SOS alerts in areas with no cellular coverage. Available as standalone devices and integrated into select Garmin outdoor watches.

A sleep disorder characterized by persistent difficulty falling asleep, staying asleep, or waking too early. Sleep-tracking wearables can help identify contributing patterns, though they are not diagnostic tools.

A condition where cells become less responsive to insulin, requiring higher levels to regulate blood sugar. A precursor to type 2 diabetes, it can be monitored through CGM data and metabolic biomarkers.

The ability of different health devices, apps, and systems to exchange and use data seamlessly. Standards like FHIR and platforms like Apple Health and Google Health Connect aim to improve interoperability.

The concentration of glucose in the fluid surrounding body cells, measured by CGMs. It typically lags 5-15 minutes behind blood glucose readings but provides valuable trend information.

A temporary sleep disorder caused by rapidly traveling across time zones, disrupting the body's circadian rhythm and causing fatigue, insomnia, and cognitive impairment. Some wearables provide jet lag advisors.

The body's ability to perceive the position and movement of its joints without visual input, a component of proprioception. Some advanced wearables and rehabilitation devices use motion sensors to assess and train this ability.

A heart rate training zone calculation that uses heart rate reserve (max HR minus resting HR) to determine target zones. It is more personalized than simple percentage-of-max methods because it accounts for individual resting heart rate.

A metabolic state where the body primarily burns fat for fuel instead of carbohydrates, producing ketone bodies. Achieved through very low carbohydrate intake or prolonged fasting and monitorable through blood, breath, or urine tests.

The exercise intensity at which lactic acid begins to accumulate in the bloodstream faster than it can be removed. Training at or near lactate threshold improves endurance performance. Garmin and other platforms estimate it from heart rate and pace data.

The total weight of all body components except fat, including muscle, bone, organs, and water. Tracking lean body mass is more informative than tracking total weight, especially during body recomposition programs.

The initial stages of non-REM sleep (stages N1 and N2) characterized by slower brain waves and reduced muscle activity. Light sleep makes up about 50-60% of total sleep time and plays a role in memory processing.

The use of bright artificial light to treat circadian rhythm disorders, seasonal affective disorder, and jet lag by influencing melatonin production and sleep timing.

The duration of an individual's life, increasingly studied alongside healthspan to emphasize quality of life in later years. VO2 max, HRV, and resting heart rate from wearables are considered biomarkers associated with longevity.

A unit measuring light intensity as perceived by the human eye. Relevant to circadian health because bright light exposure (above 10,000 lux) in the morning helps regulate the body's internal clock.

A branch of artificial intelligence where algorithms learn patterns from data without being explicitly programmed. Wearables use machine learning to improve sleep staging accuracy, activity recognition, and health anomaly detection.

The three main nutrients required in large quantities: carbohydrates, proteins, and fats. Each serves different roles in energy production, tissue repair, and hormone regulation, providing the foundation of nutritional planning.

A standard measure of accuracy for continuous glucose monitors, expressed as a percentage. Lower MARD values indicate better accuracy. Consumer CGMs typically have MARD values between 8-14%.

The highest heart rate an individual can safely achieve during maximal physical effort. Commonly estimated as 220 minus age, though individual variation is significant and wearables can detect actual max HR during intense workouts.

A hormone produced by the pineal gland in response to darkness that regulates the sleep-wake cycle. Melatonin production is suppressed by blue light exposure in the evening.

A health feature in wearables and apps that monitors cycle phases, predicts periods and fertile windows, and correlates cycle data with sleep, temperature, and HRV changes. Offered by Oura, Apple Watch, and Fitbit.

A unit representing the energy cost of physical activity as a multiple of resting metabolic rate. One MET equals approximately 3.5 mL of oxygen per kg per minute. Walking is about 3 METs, running about 8-12 METs.

The rate at which the body converts food into energy, influenced by age, sex, body composition, physical activity level, and hormonal status. It encompasses BMR, TEF, and activity-related expenditure.

An emerging display technology using microscopic self-emitting LEDs. MicroLED promises brighter, more efficient, and more durable screens than OLED and is expected to appear in future premium wearable devices.

Essential vitamins and minerals needed in small amounts for proper body function, including vitamin D, iron, magnesium, zinc, and B vitamins. Deficiencies can impact energy, recovery, and sleep quality.

A satellite positioning system that receives signals on multiple frequency bands from multiple satellite constellations. Multi-band GNSS in wearables dramatically improves accuracy in challenging environments like urban canyons and dense forests.

The weight of skeletal muscle tissue in the body, important for metabolic health, physical performance, and healthy aging. Measured by BIA, DEXA, or estimated by wearables and smart scales.

The percentage of hemoglobin in local muscle tissue that is carrying oxygen, measured non-invasively by near-infrared spectroscopy. Devices like the Moxy sensor track SmO2 for real-time training feedback.

The energy expended for all physical activities other than intentional exercise, including fidgeting, walking, standing, and household chores. NEAT can account for a significant portion of daily calorie burn.

A short-range wireless technology used in some wearables for contactless payments, quick device pairing, and scanning NFC tags. Many smartwatches support NFC-based tap-to-pay functionality.

A display setting that dims or disables a wearable's screen during sleep hours to prevent light disturbance. Some devices also reduce sensor polling frequency at night to conserve battery while maintaining sleep tracking.

Heart rate variability measured during sleep, considered the most reliable HRV reading because it eliminates confounding factors like activity, stress, and caffeine. Used by Oura and WHOOP as the basis for recovery scoring.

Describing a measurement technique that does not require breaking the skin or entering the body. Wrist-based heart rate monitoring, optical SpO2, and temperature sensing are all examples of non-invasive biometric measurements.

The stages of sleep other than REM, comprising light sleep (N1 and N2) and deep sleep (N3). NREM accounts for approximately 75-80% of total sleep time and is critical for physical restoration.

A common sleep disorder in which the upper airway repeatedly collapses during sleep, causing breathing interruptions, fragmented sleep, and drops in blood oxygen. SpO2 data from wearables can flag potential OSA.

Heart rate measurement using photoplethysmography (PPG), where LED light shone into the skin detects blood volume changes. It is the standard heart rate sensing method in modern smartwatches and fitness bands.

A daily score from 0-100 generated by the Oura Ring that indicates how recovered and prepared the body is for the day. It factors in sleep quality, HRV, resting heart rate, body temperature deviation, and recent activity.

A smart ring made by Oura Health that tracks sleep, readiness, and activity using sensors embedded in a titanium band. It measures heart rate, HRV, body temperature, and blood oxygen from the finger.

A short-term state of accumulated training fatigue that, with adequate rest, leads to performance supercompensation. Distinguished from overtraining syndrome by its reversibility with a few days of recovery.

A condition resulting from excessive training without adequate recovery, characterized by decreased performance, chronic fatigue, mood disturbances, and elevated resting heart rate. HRV monitoring can help detect early warning signs.

An imbalance between free radicals and antioxidants in the body, which can damage cells and contribute to aging and disease. Intense exercise temporarily increases oxidative stress, while chronic elevation is linked to various health conditions.

The volume of oxygen consumed by the body per minute during exercise, directly proportional to energy expenditure and a fundamental measure of aerobic capacity.

The rest-and-digest division of the autonomic nervous system that slows heart rate, promotes digestion, and facilitates recovery. Dominates during relaxation, sleep, and is reflected in higher HRV values.

The systematic planning of training into cycles of varying intensity and volume to optimize performance gains, manage fatigue, and prevent overtraining over weeks, months, or seasons.

An optical sensing technique that shines LED light into the skin and measures blood volume changes to determine heart rate, SpO2, and respiratory rate. PPG is the core technology behind most wrist-based biometric measurements.

A comprehensive clinical sleep study that records brain waves, eye movements, muscle activity, heart rhythm, and breathing during sleep. PSG is the gold standard for diagnosing sleep disorders and is performed in a sleep laboratory.

A device that directly measures mechanical power output during cycling or running in watts, enabling precise and objective training intensity quantification unaffected by external conditions.

A non-invasive method of measuring blood oxygen saturation (SpO2) by passing red and infrared light through tissue and analyzing the ratio of oxygenated to deoxygenated hemoglobin. Integrated into most modern wearables.

The speed at which the blood pressure waveform travels through the arterial system, used as a marker of arterial stiffness and cardiovascular health. Withings devices can estimate PWV alongside blood pressure readings.

The combination of three graphical deflections on an ECG that represents the electrical depolarization of the heart's ventricles. Accurate QRS detection is essential for heart rate and HRV measurement in both clinical and consumer devices.

A subjective scale (typically 1-10) used to gauge how hard an exercise feels. Some training platforms combine RPE self-reports with objective wearable data for a more complete picture of training intensity.

A composite metric from wearables that indicates how prepared the body is for physical and mental demands, combining overnight recovery data such as HRV, resting heart rate, and sleep quality with long-term trends.

A metric calculated by wearables such as WHOOP that quantifies how well the body has recovered from prior strain, derived from sleep performance, HRV, resting heart rate, and respiratory rate.

Rapid Eye Movement sleep, the stage associated with vivid dreaming, emotional processing, and memory consolidation. Characterized by rapid eye movements and near-complete muscle paralysis, REM typically makes up 20-25% of total sleep.

The number of breaths taken per minute, normally 12-20 at rest for healthy adults. Wearables estimate respiratory rate from HRV and accelerometer data, and deviations from baseline can indicate illness or stress.

The number of heartbeats per minute while at complete rest, ideally measured upon waking. Lower RHR generally indicates better cardiovascular fitness, with typical adult values of 60-100 bpm and trained athletes below 50 bpm.

The number of calories burned at rest in a neutral environment after a 12-hour fast. Slightly higher than BMR, RMR is used to estimate daily calorie needs for nutrition planning.

Root Mean Square of Successive Differences between heartbeats, the most common time-domain measure of HRV. Higher rMSSD values indicate greater parasympathetic (vagal) activity and is the primary HRV metric in most consumer wearables.

A set of biomechanical metrics measured during running, including cadence, ground contact time, vertical oscillation, stride length, and left-right balance. Garmin and Polar offer running dynamics through chest straps or pods.

Samsung's smartwatch line featuring health sensors including optical heart rate, ECG, BIA body composition, blood pressure monitoring, and advanced sleep tracking on the Wear OS platform.

The age-related progressive loss of skeletal muscle mass, strength, and function, typically beginning after age 30 and accelerating after 60. Regular body composition tracking and resistance training can help counteract it.

Garmin's intelligent satellite mode selection technology that automatically switches between multi-band and single-band GPS to optimize battery life while maintaining acceptable positional accuracy.

Standard Deviation of Normal-to-Normal heartbeat intervals, an HRV metric reflecting overall autonomic nervous system variability. Unlike rMSSD, SDNN captures both sympathetic and parasympathetic influence and is best measured over longer periods.

The temperature measured at the surface of the skin, tracked by wearables like the Oura Ring to detect illness, monitor menstrual cycles, assess recovery patterns, and observe circadian rhythm alignment.

A disorder characterized by repeated interruptions of breathing during sleep, including obstructive (airway collapse) and central (brain signaling) types. Some wearables flag potential sleep apnea through SpO2 variability patterns.

The overall structure and pattern of sleep stages throughout the night, including the proportion and timing of light, deep, and REM sleep cycles. Healthy architecture shows progressively more REM as the night progresses.

The cumulative effect of not getting enough sleep over time. Chronic sleep debt impairs cognitive function, immune health, metabolic regulation, and emotional well-being, and cannot be fully repaid with a single long sleep.

The percentage of time spent actually sleeping while in bed, calculated as total sleep time divided by total time in bed. Above 85% is generally considered good sleep efficiency.

A set of behavioral practices and environmental conditions that promote consistent, high-quality sleep, including consistent bedtimes, cool room temperature, darkness, and limiting screen exposure before bed.

The amount of time it takes to transition from full wakefulness to the onset of sleep. Normal sleep latency is 10-20 minutes; shorter may indicate sleep deprivation, while longer may suggest insomnia.

A composite metric that combines multiple sleep parameters (duration, efficiency, stages, disturbances) into a single 0-100 score. Companies like Oura, Fitbit, Withings, and Garmin each use proprietary scoring algorithms.

The process of classifying sleep into its component stages (wake, light, deep, REM) using sensor data. Consumer wearables approximate clinical staging using heart rate, HRV, movement, and sometimes SpO2.

Another name for deep sleep (N3 stage), characterized by high-amplitude delta brain waves. Essential for physical recovery, growth hormone release, and immune function.

A finger-worn wearable device equipped with sensors to track health metrics. Smart rings like the Oura Ring, Ultrahuman Ring Air, and Samsung Galaxy Ring offer discreet monitoring with strong vascular signal quality from the finger.

A Wi-Fi or Bluetooth-connected scale that measures weight along with body composition metrics using bioimpedance analysis. Brands like Withings and Garmin offer smart scales that sync data to their health platforms.

The discrepancy between the body's natural sleep schedule (chronotype) and socially imposed schedules, common when weekend sleep patterns differ significantly from weekday patterns.

A power management feature found in select Garmin watches that uses a photovoltaic cell beneath the watch crystal to harvest ambient light energy, significantly extending battery life during outdoor use.

A measurement of the percentage of oxygen-saturated hemoglobin relative to total hemoglobin in peripheral blood. Healthy values are typically 95-100%. Consistently low readings may warrant evaluation for conditions like sleep apnea.

A WHOOP metric that quantifies the cardiovascular load experienced during a day or workout on a scale of 0-21. It is calculated from heart rate data and helps users balance training stress with recovery capacity.

A popular social fitness platform focused on running and cycling that integrates with virtually all major wearable brands. Strava provides route tracking, segment leaderboards, training analysis, and a social feed.

A wearable-derived metric that estimates physiological stress based on HRV analysis and other signals. Garmin, Fitbit, and Samsung all offer versions of continuous stress monitoring throughout the day.

The amount of blood pumped by the left ventricle of the heart with each beat. Stroke volume increases with cardiovascular fitness and, combined with heart rate, determines cardiac output.

Fat stored directly beneath the skin, making up the majority of body fat. Less metabolically harmful than visceral fat but contributing to overall body fat percentage.

A training principle where the body recovers beyond its previous baseline after adequate rest following a training stimulus, resulting in improved performance capacity over time.

The master circadian clock located in the hypothalamus that coordinates the body's 24-hour biological rhythms based on light signals received from the eyes.

The fight-or-flight division of the autonomic nervous system that increases heart rate, blood pressure, and energy mobilization in response to stress or physical demands. Elevated activity at rest may indicate incomplete recovery.

A resting heart rate above 100 beats per minute. Can be caused by fever, anxiety, medication, dehydration, or underlying cardiac conditions requiring medical evaluation.

The total number of calories burned per day, combining basal metabolic rate, the thermic effect of food, and all physical activity. Wearables estimate TDEE to help guide nutrition and weight management.

The continuous or periodic monitoring of body or skin temperature using wearable sensors. Temperature trends can indicate illness onset, menstrual cycle phases, recovery status, and circadian rhythm alignment.

The energy required to digest, absorb, and metabolize food, accounting for approximately 10% of total daily energy expenditure. Protein has the highest thermic effect among macronutrients.

A Garmin metric that rates the impact of a workout on aerobic and anaerobic fitness on a scale of 0 to 5, considering exercise intensity, duration, and the user's current fitness level.

A quantified measure of the cumulative physiological stress from training over 7-28 days. Wearables calculate training load from workout duration, intensity, and EPOC to help athletes manage fatigue and peak performance.

A Garmin metric that combines sleep history, recovery time, HRV status, training load, and stress to recommend how hard a user should train on a given day, presented as a score from 0-100.

A Garmin feature that evaluates the overall direction of fitness by analyzing training load and VO2 max trends. Categories include Productive, Peaking, Recovery, Unproductive, Detraining, and Overreaching.

Biological rhythms shorter than 24 hours, such as the 90-minute sleep cycles that alternate between NREM and REM stages throughout the night.

A continuous glucose monitoring platform by Ultrahuman that combines a CGM sensor with an app providing metabolic scores and insights, helping users understand how food, exercise, and sleep affect glucose levels.

Smart nutritional supplements by Ultrahuman designed to be guided by metabolic data from their CGM and Ring Air platforms, representing the integration of wearable data with personalized nutrition recommendations.

A smart ring by Ultrahuman that tracks sleep, movement, and recovery metrics using PPG sensors in a lightweight titanium design, providing insights through the Ultrahuman app.

A measure of the strength of ultraviolet radiation from the sun at a particular place and time. Some outdoor-focused wearables display the UV index to help users manage sun exposure and reduce skin damage risk.

The activity level of the vagus nerve, the primary component of the parasympathetic nervous system. High vagal tone is associated with better heart rate regulation, lower inflammation, and improved stress resilience. HRV is the most accessible indirect measure.

An estimate of the age of a person's blood vessels based on arterial stiffness and cardiovascular markers. Withings BPM devices provide vascular age estimates alongside blood pressure readings. A vascular age higher than chronological age indicates elevated cardiovascular risk.

The degree of vertical bounce in a runner's stride, measured in centimeters. Less vertical oscillation generally indicates more efficient running form and less wasted energy.

Fat stored deep in the abdominal cavity surrounding vital organs. Higher levels are strongly associated with increased risk of cardiovascular disease, diabetes, and metabolic syndrome. Smart scales often provide visceral fat ratings.

The maximum rate of oxygen consumption during incremental exercise, expressed in mL/kg/min. Considered the gold standard measure of cardiorespiratory fitness and a strong predictor of cardiovascular health and longevity.

A measurement comparing waist circumference to hip circumference, used as an indicator of central obesity and associated health risks. Ratios above 0.90 for men or 0.85 for women indicate increased cardiovascular risk.

A classification indicating a wearable's ability to withstand water exposure, measured in atmospheres (ATM) or meters. A 5 ATM rating is suitable for swimming, while IP68 ratings indicate dust and water resistance.

An electronic device designed to be worn on the body that collects data about the wearer's health, fitness, or environment. Common form factors include watches, bands, rings, chest straps, and patches.

A feature in the WHOOP app that allows users to log daily behaviors such as caffeine intake, alcohol, screen time, and supplements. The platform then correlates journal entries with recovery and sleep to identify impactful habits.

WHOOP's daily recovery score from 0-100%, derived from HRV, resting heart rate, respiratory rate, and sleep performance. Green (67-100%) suggests readiness for strain, yellow (34-66%) indicates moderate readiness, and red (0-33%) recommends rest.

A screen-free fitness wearable focused on recovery, strain, and sleep optimization. WHOOP operates on a subscription model and is popular among athletes for its detailed analytics and coaching recommendations.

A Wi-Fi-connected smart scale by Withings that measures weight, body fat, water percentage, muscle mass, and bone mass using bioimpedance analysis, syncing automatically to the Withings Health Mate app.

A hybrid smartwatch by Withings that combines an analog watch design with medical-grade ECG and SpO2 sensors. It has received FDA clearance for AFib detection and clinical-grade pulse oximetry.

Heart rate measurement taken from the wrist using optical PPG sensors. While convenient, wrist-based readings can be less accurate than chest straps during high-intensity or high-motion activities due to motion artifacts.

A technology using two X-ray beams at different energy levels to measure tissue composition. The clinical application, DEXA (Dual-Energy X-Ray Absorptiometry), is used to assess bone density and body composition with high precision.

A mind-body practice involving physical postures, breathing techniques, and meditation. Most wearables include a dedicated yoga activity profile that tracks duration, heart rate, and estimated calories burned during sessions.

An environmental cue, such as light, temperature, or meal timing, that synchronizes the body's circadian rhythm to the 24-hour day-night cycle. Light is the strongest zeitgeber, making controlled light exposure a key tool for managing sleep timing.

A durable, scratch-resistant ceramic material made from zirconium dioxide, used in the construction of premium wearables. The Apple Watch Ultra and some smart rings use zirconia ceramic for its hardness, biocompatibility, and light weight.

Low-intensity aerobic exercise performed at 60-70% of maximum heart rate, targeting fat oxidation and building mitochondrial density. Widely recommended for building a strong aerobic base and improving metabolic health.