Reference
Sleep Science Glossary
The key terms behind every calculator on this site, explained in plain language — each one links to the tool that measures it.
Sleep debt
The cumulative gap between the sleep you need and the sleep you actually get, measured in hours.
Sleep debt builds night by night: if your body needs 8 hours and you get 6.5, that's 1.5 hours of debt added to whatever you were already carrying. A single long night's sleep can repay a small amount, but a debt built up over weeks generally takes more than one night to clear. Chronic sleep debt is linked to impaired attention, mood changes, weakened immune response, and metabolic effects.
Calculate your own sleep debt →Sleep cycle
A recurring loop through light sleep, deep sleep, and REM sleep that repeats several times per night.
The commonly cited figure is 90 minutes per cycle, but a 2024 study analyzing over 6,000 polysomnography-recorded cycles found a median length of 96 minutes, with individual cycles ranging from about 70 to 120 minutes. Most adults complete 4–6 cycles per night, with deep sleep concentrated in earlier cycles and REM sleep growing longer later in the night.
Calculate your own sleep cycle →REM sleep
The sleep stage most associated with dreaming, emotional processing, and memory consolidation.
REM (rapid eye movement) sleep is marked by fast, low-amplitude brain activity resembling wakefulness, paired with temporary muscle paralysis that prevents you from acting out dreams. It dominates the later sleep cycles of the night, which is why cutting sleep short reduces REM disproportionately.
Calculate your own rem sleep →Deep sleep
Slow-wave (N3) sleep — the stage most responsible for physical recovery and immune regulation.
Deep sleep is characterized by slow, high-amplitude brain waves and is the hardest stage to wake someone from. It's when the body does most of its physical repair work — tissue growth, immune regulation, and clearance of metabolic waste from the brain — and it's concentrated in the earlier cycles of the night.
Calculate your own deep sleep →Circadian rhythm
Your internal ~24-hour clock, anchored by light, food, and temperature cues.
Your circadian rhythm governs far more than sleep timing — it influences body temperature, hormone release, and alertness across the day. Morning light is the strongest anchor for keeping it aligned with the actual day-night cycle; irregular schedules and evening light exposure are the most common causes of misalignment.
Calculate your own circadian rhythm →Chronotype
Your natural tendency toward morning or evening alertness.
Chronotype is influenced by genetics and shifts with age — adolescents typically skew later, older adults earlier. It's commonly grouped into 'morning types' (larks), 'evening types' (owls), and intermediate types, though real chronotype exists on a continuum rather than in strict categories.
Calculate your own chronotype →Sleep efficiency
The percentage of time in bed actually spent asleep.
Calculated as total sleep time divided by time in bed, times 100. Healthy adults typically score 85% or higher; below 85% often signals fragmented sleep, and sustained scores above roughly 95% combined with daytime fatigue can itself be a sign of significant sleep deprivation.
Calculate your own sleep efficiency →Sleep latency
How long it takes to fall asleep after getting into bed.
Also called sleep onset latency. Under 20 minutes is typical for most adults; consistently taking over 30 minutes is one of the more concrete signs of a sleep-onset problem. Somewhat counterintuitively, regularly falling asleep in under 5 minutes can itself indicate significant sleep debt rather than being a good sign.
Calculate your own sleep latency →Insomnia
Persistent difficulty falling or staying asleep despite adequate opportunity to sleep.
Clinically, insomnia disorder requires symptoms at least 3 nights a week for at least 3 months, along with real daytime impact on mood, energy, or focus. The most widely used clinical severity measure is the Insomnia Severity Index (ISI), a copyrighted, licensed instrument — see our full explainer on its scoring, validity, and how to access it.
Calculate your own insomnia →Sleep apnea risk
The likelihood of repeated breathing interruptions during sleep that fragment rest.
Obstructive sleep apnea causes brief, repeated pauses in breathing that reduce oxygen levels and fragment sleep, often without the person waking enough to remember it. Screening questionnaires like STOP-BANG look at snoring, observed pauses, tiredness, blood pressure, and body measurements — a self-screener can flag risk, but only a sleep study can diagnose it.
Calculate your own sleep apnea risk →Melatonin
The hormone that signals your body it's time to sleep.
Melatonin release is triggered by darkness and suppressed by light, particularly blue-spectrum light in the evening. As a supplement, timing and dose both matter — lower doses (0.5–3mg) taken 1–2 hours before target bedtime are generally more effective for shifting sleep timing than large doses taken right before bed.
Calculate your own melatonin →Jet lag
Circadian misalignment from crossing time zones faster than your internal clock can adjust.
The body's clock typically shifts by about an hour per day when adapting to a new time zone, so a long-haul flight can take most of a week to fully adjust to. Eastward travel (advancing your schedule) is generally harder to adjust to than westward travel.
Calculate your own jet lag →Sleep hygiene
The habits and environment that support consistent, high-quality sleep.
This includes light exposure timing, room temperature, noise control, screen use before bed, and caffeine/alcohol timing. Sleep hygiene changes alone usually aren't sufficient to resolve clinical insomnia, but they're a reasonable first step and a necessary complement to treatments like CBT-I.
Calculate your own sleep hygiene →Power nap
A short, strategically timed nap that reduces sleepiness without causing grogginess.
Typically 10–20 minutes, timed to stay within light sleep and avoid the deep slow-wave sleep that causes grogginess (sleep inertia) on waking. Naps longer than about 30 minutes risk entering deep sleep and can leave you feeling worse immediately afterward, even though they may still repay some sleep debt.
Calculate your own power nap →Caffeine half-life
The roughly 5–6 hours it takes your body to clear half a dose of caffeine.
Half-life varies by individual — smokers tend to clear caffeine faster, while pregnancy and certain medications slow it — but 5–6 hours is a reasonable average. A 200mg coffee at 3pm still leaves roughly 100mg active at 9pm, enough to measurably delay sleep onset for many people.
Calculate your own caffeine half-life →Frequently asked questions
What's the difference between sleep debt and sleep deprivation?+
Sleep debt is the cumulative, measurable gap between the sleep you need and what you've gotten, typically tracked over days or weeks. Sleep deprivation more commonly refers to the state of currently not having enough sleep, whether from one bad night or a long-running debt.
Is REM sleep or deep sleep more important?+
Both matter for different things — deep sleep drives physical recovery and immune function, REM sleep drives emotional processing and memory consolidation. Neither substitutes for the other, and cutting sleep short tends to reduce REM disproportionately since it's concentrated later in the night.
How is this glossary different from a general dictionary definition?+
Each definition is written specifically around how the concept shows up in sleep tracking, and links to a free calculator that lets you apply it to your own numbers rather than just read about it.
Social jet lag
The mismatch between your body clock and your social schedule.
A common example is sleeping much later on weekends than weekdays — the same kind of circadian disruption as crossing time zones, just self-imposed. Research links chronic social jet lag to some of the same metabolic and mood effects associated with real jet lag and shift work.
Calculate your own social jet lag →