Glossary

Every technical term, in one plain sentence.

Sleep medicine has a lot of jargon. Here is all of it that appears on this site, said simply. Underlined words elsewhere on the site show these same definitions where you read them.

24 terms

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ActigraphyMeasurement: A wrist sensor that estimates sleep and wake from movement.; A small motion sensor worn on the wrist. It tracks when you move and when you stay still, which gives a good estimate of when you fell asleep and woke up over many nights. It is less exact than a lab study, but it can run for weeks at home.
AdenosineBiology: The chemical that builds up while you are awake and creates sleep pressure.; The longer you stay awake, the more of it accumulates, and the sleepier you feel. Sleep clears it. Caffeine works by blocking its signal, which is why the pressure is still there when the caffeine wears off.
AHICare & treatment: The number of breathing interruptions per hour of sleep.; Apnea-hypopnea index. Under 5 is normal, 5 to 15 mild, 15 to 30 moderate, above 30 severe. It is the standard number used to grade sleep apnea and to judge whether treatment is working.; Also called: apnea-hypopnea index
Baseline nightData & risk: Your untouched first night, recorded before anything is changed.; Nothing is adjusted on the first night. It is the reference every later night is compared against, which is what turns the rest of the week into a measurement rather than an impression.
C-indexData & risk: A score for how well a model ranks who is at higher risk. 0.5 is a coin flip.; Concordance index. Given two people, it is the chance the model correctly says which one develops the condition sooner. 0.5 means guessing, 0.7 is clinically useful, 0.8 and above is strong. It measures ranking, not certainty about any one person.; Also called: concordance index
CBT-ICare & treatment: The first-line treatment for chronic insomnia, and it is not a drug.; A short, structured program that changes sleep timing, time in bed and the thinking patterns that keep people awake. Major guidelines recommend it before sleeping pills because the gains hold after the program ends.; Also called: cognitive behavioral therapy for insomnia
ChronotypeBiology: Whether your body clock naturally runs early or late.; Some people are wired to sleep and wake early, others late. It is largely genetic and shifts with age. Fighting your chronotype every weekday is one of the most common hidden causes of poor sleep.
Circadian rhythmBiology: Your internal 24-hour clock, set mainly by light.; A body-wide daily timer that decides when you feel alert, when your temperature drops and when sleep hormones are released. Morning light pulls it earlier, bright evening light pushes it later. Most modern sleep problems start here.; Also called: circadian, body clock
CortisolBiology: The alerting hormone that should peak in the morning, not at night.; Part of the wake-up signal. It normally rises sharply just before you get up and falls through the evening. Stress, late work and irregular hours can flatten or shift that curve, which makes falling asleep and waking up both harder.
EEGMeasurement: A recording of the brain's electrical activity, used to tell sleep stages apart.; Small sensors on the scalp pick up the brain's electrical rhythms. The shape and speed of those waves is how deep sleep, light sleep and REM are told apart minute by minute.; Also called: electroencephalography
Homeostatic sleep driveBiology: The pressure to sleep that grows the longer you stay awake.; One of the two systems that set your sleep. It builds through the day and discharges in deep sleep. Naps and late caffeine both change how much pressure is left at bedtime.; Also called: homeostatic, sleep pressure
HRVMeasurement: The small variation in time between heartbeats, a marker of recovery.; Healthy hearts do not tick like a metronome. The gap between beats shifts slightly. More variation overnight usually means the calming side of the nervous system is in charge, which is what recovery looks like. Low variation often follows stress, alcohol, illness or a late meal.; Also called: heart rate variability
HypnogramMeasurement: A chart of your sleep stages across the night, from lights out to wake.; A stepped line showing which stage you were in at each moment: awake, light, deep or REM. It makes the shape of a night visible at a glance, including how often you surfaced.
MelatoninBiology: The hormone that tells your body night has arrived.; Released by the brain as light fades. It does not knock you out. It signals timing, which is why bright evening light delays it and why the dose and hour matter more than the amount.
OximetryMeasurement: A finger or wrist sensor that measures how much oxygen your blood is carrying.; A light shone through the skin estimates blood oxygen. Repeated dips through the night are one of the clearest signs that breathing is being interrupted during sleep.; Also called: pulse oximetry, SpO2
PolysomnographyMeasurement: The full overnight sleep study: brain waves, breathing, oxygen, heart rate and movement recorded together.; The reference test in sleep medicine. Sensors record brain activity, eye movement, muscle tone, airflow, chest effort, blood oxygen and heart rhythm through the night. Read together, they show your sleep stages and any breathing problems.; Also called: PSG, sleep study
REM sleepSleep stages: The dreaming stage, important for memory and emotional balance.; Rapid eye movement sleep. The brain becomes highly active while the body stays still. It handles emotional processing and memory consolidation, and it builds up in the second half of the night, which is why cutting sleep short mostly cuts REM.; Also called: rapid eye movement
Sleep apneaCare & treatment: Repeated pauses in breathing during sleep that fragment the night.; The airway narrows or closes while you sleep, oxygen drops and the brain briefly wakes you to restart breathing. This can happen dozens of times an hour without being remembered. It is common, underdiagnosed and treatable, and it raises heart and metabolic risk when left alone.; Also called: apnea, obstructive sleep apnea, OSA
Sleep architectureSleep stages: The order and amount of each sleep stage across the night.; A normal night cycles through light, deep and REM roughly every ninety minutes, with deep sleep loaded early and REM loaded late. When that order is disturbed, the total hours can look normal while recovery is not.
Sleep efficiencyMeasurement: The share of time in bed that you were actually asleep.; Time asleep divided by time in bed. Around 85 percent or higher is generally considered solid. Low efficiency usually means long time to fall asleep, frequent awakenings, or simply spending too many hours in bed.
Sleep latencyMeasurement: How long it takes you to fall asleep after lights out.; Measured from the moment you try to sleep to the first stable sleep. Very long times point to a wound-up nervous system or a body clock that is not ready yet. Very short times can be a sign of real sleep debt.; Also called: sleep onset
Slow-wave sleepSleep stages: The deepest sleep, when the body does most of its physical repair.; Named for the large, slow brain waves that appear in it. Growth hormone release, tissue repair and the brain's overnight clearing process are concentrated here. It sits mostly in the first half of the night and is the first thing lost to heat, alcohol and noise.; Also called: deep sleep, SWS
ThermoregulationBiology: How your body manages its temperature, which has to fall for sleep to start.; Core temperature drops about one degree as you fall asleep, and the body releases heat through the hands and feet to make that happen. A warm room, a hot mattress or heavy bedding can block the drop and stall sleep entirely.
WASOMeasurement: The total time you spend awake after first falling asleep.; Adds up every awakening between falling asleep and getting up. It is often the number that matters most for how broken a night feels, even when total sleep time looks fine.; Also called: wake after sleep onset

These definitions are written for general understanding. They are not medical advice and do not replace a diagnosis or a conversation with your own physician.