Every sleep word, in one paragraph

Actigraphy is sleep measurement by movement. A sensor records motion minute by minute, and an algorithm scores each minute as sleep or wake from how still you were. It estimates sleep rather than observing the brain directly.

Blue light is the short wavelength end of visible light, the part the body clock is most sensitive to. Light in the evening delays that clock, but total brightness and timing matter more than whether a screen looks blue.

Your chronotype is where your body clock naturally sits: early, late, or somewhere in the middle. It shows up as the bedtime and wake time you drift towards with no alarm and no obligations, and it shifts across your life.

Your circadian rhythm is the roughly 24 hour internal cycle that decides when you feel sleepy and when you feel alert. It is run by a master clock in the brain, and light is the strongest signal keeping it lined up with the real day.

The cortisol awakening response is the normal rise in cortisol over roughly the first 30 to 45 minutes after you wake up. It is part of how the body switches the day on, and it happens whatever time you wake.

Dawn simulation is a light that brightens gradually in the half hour or so before your alarm, imitating sunrise so you wake into light rather than darkness. The evidence for it is mixed, and reported effects are usually modest.

Deep sleep is N3, the slow wave stage where brain waves are largest and slowest and you are hardest to wake. Most of it happens in the first half of the night, and it is the stage most tied to feeling physically restored.

A hypnogram is the stepped chart that plots sleep stages against time, so a whole night reads left to right in one picture. Wake sits at the top and deep sleep at the bottom, and each block shows how long a stage lasted.

Melatonin is the hormone your brain releases in the evening to signal biological night. It does not knock you out, it tells the rest of your body what time it is, and light in the evening delays its release.

A nocturnal awakening is any wake up between falling asleep and your final rise time. Brief ones happen between cycles all night and are mostly forgotten by morning. They get more frequent in the second half of the night.

NREM sleep is everything that is not REM, split into three stages. N1 is the drift at sleep onset, N2 is the bulk of the night, and N3 is deep slow wave sleep. Together they make up most of an adult night.

Polysomnography is the full overnight sleep study, run in a lab or at home with a technician kit. It records brain waves, eye movement, muscle tone, breathing, oxygen and heart rate, and it is the only way sleep stages are actually measured.

REM sleep is the stage where your eyes move quickly under closed lids, your brain is close to awake, and most vivid dreaming happens. Your body is temporarily paralysed. REM periods get longer as the night goes on.

Sleep apnea is a disorder where breathing repeatedly stops and restarts during sleep. In the obstructive form the airway collapses, oxygen drops, and the brain briefly wakes you to reopen it. Only a doctor can diagnose it.

Sleep architecture is the overall structure of a night: which stages you pass through, in what order, for how long, and how often you surface. It is read off a hypnogram rather than judged from any single number.

Sleep debt is the running gap between the sleep you need and the sleep you have actually had, usually totted up across the past week or two. Small shortfalls add up, and they show as slower reactions before they show as feeling tired.

Sleep efficiency is total sleep time divided by time in bed, written as a percentage. If you were in bed eight hours and slept seven, that is roughly 88%. Around 85% or higher is the conventional benchmark for adults.

Sleep hygiene is the set of everyday habits that make sleep easier: consistent timing, a dark cool quiet room, light in the morning, caffeine and alcohol earlier, and a wind-down. It is a floor, not a treatment.

Sleep inertia is the fogged, slowed down state right after waking, when reaction time and judgement are still below normal. It usually clears within 15 to 30 minutes, and it is worst when you wake out of deep sleep.

Sleep latency is the time from lights out to the moment you actually fall asleep. Roughly 10 to 20 minutes is typical for an adult. Consistently dropping off in under five minutes usually means you are short on sleep.

Sleep onset is the moment you cross from wake into sleep. In a lab it is the first epoch scored as sleep after lights out. It is a transition rather than a switch, and most people misremember when it happened.

Sleep pressure is the drive to sleep that builds the longer you stay awake and drains while you sleep. In the two process model it is process S, and adenosine accumulating in the brain is the accepted mechanism behind it.

The sleep regularity index, or SRI, is a score from 0 to 100 for how likely you are to be in the same state, asleep or awake, at the same clock time on two different days. 100 is a perfectly repeated schedule.

A sleep spindle is a short burst of fast brain activity, usually about half a second to three seconds long, that appears in stage N2. Spindles are one of the markers used to score N2, and they are linked to memory consolidation.

Snoring is the sound of soft tissue vibrating in a narrowed airway as you breathe during sleep. It is common and often harmless, but loud snoring with pauses in breathing is a reason to talk to a doctor.

Social jetlag is the gap between the sleep your body clock wants and the sleep your schedule allows, measured as the shift in your mid sleep time between work days and free days. Two hours of it is like flying two time zones every week.

Time in bed, or TIB, is the whole window from lights out to getting up, awake minutes included. It is the denominator of sleep efficiency, so widening it to chase more sleep usually lowers the percentage instead.

Total sleep time, or TST, is how much you actually slept in a night, once the time you spent awake in bed is taken out. It is time in bed minus sleep latency and minus wake after sleep onset.

Wake after sleep onset, or WASO, is the total number of minutes you are awake between falling asleep and getting up for the day. It counts minutes, not the number of wakings, and it pulls sleep efficiency down.

The wake maintenance zone is the stretch of a few hours before your usual bedtime when your body clock pushes hardest for alertness. It is the hardest part of the day to fall asleep in, even when you are genuinely tired.

White, pink and brown noise are steady sounds that differ in how their energy is spread across frequencies. White is flat, pink falls off gently and sounds balanced, brown falls faster and sounds like a low rumble.