Sleep Glossary
Fifty sleep terms explained in short, plain language, from adenosine to zeitgeber.
On this page
Plain language definitions of sleep terms: sleep cycle, REM, sleep inertia, chronotype, circadian rhythm, sleep latency, sleep debt, sleep efficiency and more.
Sleep glossary: plain language definitions
Sleep science comes with a lot of vocabulary. This glossary explains the terms used across the calculators and guides, with short definitions you can scan. Terms are listed alphabetically.
Jump to: A B C D E H I J M N P R S T U W Z
A to Z of sleep terms
- Actigraphy
- A method that estimates sleep and wake from wrist movement over days or weeks. Many consumer trackers use a similar idea.
- Adenosine
- A chemical that builds up in the brain while you are awake and contributes to sleep pressure. Caffeine blocks its receptors, which is why it makes you feel less sleepy.
- Biphasic sleep
- A pattern with two sleep periods in 24 hours, for example a night sleep and an afternoon nap.
- Caffeine half life
- The time it takes the body to clear half of the caffeine. It is commonly about 5 to 6 hours in healthy adults but varies widely.
- Chronotype
- Your natural tendency toward earlier or later sleep and activity, often described as morning type or evening type.
- Circadian rhythm
- An internal cycle of about 24 hours that shapes sleepiness, alertness, body temperature and hormone release.
- Cortisol awakening response
- A rise in the hormone cortisol in the first 30 to 45 minutes after waking that helps the body become alert.
- Deep sleep (N3)
- The deepest stage of NREM sleep, also called slow wave sleep. It is more common in the first half of the night.
- Delayed sleep phase
- A pattern where the body clock runs late, so sleep and waking happen much later than usual. It is common in teenagers and can also be a clinical condition.
- Dim light melatonin onset
- The time in the evening when the body begins to release melatonin under dim light, a marker of the body clock.
- EEG
- Electroencephalography, a recording of brain electrical activity that sleep labs use to identify sleep stages.
- Homeostatic sleep drive
- Another name for sleep pressure, the growing need for sleep the longer you stay awake.
- Hypnic jerk
- A sudden muscle twitch that can happen as you fall asleep. It is common and usually harmless.
- Insomnia
- Difficulty falling asleep, staying asleep or waking too early despite enough opportunity to sleep, with daytime effects. Long lasting insomnia should be assessed by a doctor.
- Jet lag
- A temporary mismatch between your body clock and the local time after crossing time zones. Many travelers adjust about one hour of time difference per day.
- Melatonin
- A hormone released in darkness that signals night to the body. Its timing matters more than its presence.
- Microsleep
- A brief lapse into sleep lasting a few seconds, often without awareness, which is dangerous while driving.
- N1 sleep
- The lightest stage of NREM sleep, lasting a few minutes as you drift off.
- N2 sleep
- Light sleep that takes up the largest share of the night, marked by sleep spindles and K complexes on an EEG.
- Nap
- A short sleep during the day. Short naps of about 10 to 20 minutes are often used for a quick alertness boost.
- NREM sleep
- Non rapid eye movement sleep, made up of stages N1, N2 and N3.
- Polyphasic sleep
- A pattern with several sleep periods in 24 hours. Claims of large time savings are not supported by good evidence.
- REM sleep
- Rapid eye movement sleep, when vivid dreaming is most common. REM periods get longer toward morning.
- REM rebound
- Extra REM sleep after a period of REM loss, for example after sleep deprivation or alcohol.
- Sleep architecture
- The structure of a night of sleep: how long you spend in each stage and how the stages cycle.
- Sleep consistency
- How regular your sleep and wake times are from day to day.
- Sleep cycle
- A repeating pattern through NREM and REM sleep lasting about 90 minutes on average, varying between people.
- Sleep debt
- The running shortfall between the sleep you need and the sleep you get.
- Sleep efficiency
- Time asleep divided by time in bed, shown as a percentage. About 85 percent or higher is often considered good.
- Sleep hygiene
- Habits and bedroom conditions that support good sleep, such as a steady schedule, a dark cool room and limited late caffeine.
- Sleep inertia
- The grogginess and slowed thinking right after waking, which usually fades within about 15 to 30 minutes.
- Sleep latency
- How long it takes to fall asleep after lights out. Calculators often use 14 or 15 minutes.
- Sleep midpoint
- The halfway point between falling asleep and waking, used to compare sleep timing between days.
- Sleep onset
- The moment you fall asleep.
- Sleep opportunity
- The time you set aside for sleep, from lights out to final alarm.
- Sleep pressure
- The build up of the need for sleep while you are awake, partly driven by adenosine.
- Sleep restriction
- Deliberately limiting time in bed, used in some insomnia therapies under guidance, or the unintended cutting of sleep by busy schedules.
- Slow wave sleep
- Deep NREM sleep with slow, large brain waves. It supports physical recovery and is easiest to see early in the night.
- The gap between sleep timing on work days and free days, which can feel like jet lag without any travel.
- Suprachiasmatic nucleus
- A small region in the hypothalamus that acts as the master clock for circadian rhythms.
- Total sleep time
- The actual time you spend asleep, excluding time spent awake in bed.
- Time in bed
- The time between getting into bed to sleep and getting out in the morning.
- Ultradian rhythm
- A rhythm shorter than 24 hours, such as the roughly 90 minute sleep cycle.
- Wake after sleep onset
- The total time you are awake after first falling asleep, often shortened to WASO.
- Wake window
- How long a baby or child can comfortably stay awake between sleeps, which grows with age.
- Zeitgeber
- A German word for time giver. An outside cue, most powerfully morning light, that sets the body clock.
Frequently asked questions
What is a sleep cycle?
A sleep cycle is a repeating pattern of NREM and REM sleep that lasts about 90 minutes on average. Adults usually go through four to six cycles a night.
What is sleep inertia?
Sleep inertia is the grogginess you feel right after waking. It usually fades within 15 to 30 minutes and is stronger after deep sleep or short sleep.
What is a chronotype?
A chronotype is your natural tendency to prefer earlier or later sleep times. People are often described as morning types, evening types or somewhere in between.
What is sleep latency?
Sleep latency is the time it takes to fall asleep after lights out. Many healthy adults take about 10 to 20 minutes.
Is 6 hours of sleep enough?
For most adults it is below the recommended 7 hours. Some people feel fine for a while, but research on chronic sleep restriction found that performance slips even when people do not feel very sleepy. Treat 6 hours as a short night and plan recovery sleep.
How do I adjust to daylight saving time?
Move your schedule by 15 to 20 minutes over several days before the change and get morning light afterwards. Expect a few days of adjustment.
Will waking at the end of a cycle remove morning grogginess?
It can help some people, because waking from light sleep is usually easier than waking from deep sleep. The effect is not guaranteed. Grogginess, known as sleep inertia, usually fades within 15 to 30 minutes and also depends on how much sleep you had and when you wake in your body clock.
Is 8 hours too much sleep?
Eight hours sits inside the normal range. If you regularly need 10 or more hours and still feel unrefreshed, mention it to a doctor, because it can be linked with health conditions, medicines or poor quality sleep.