Sleep latency The time between getting into bed and falling asleep.
Healthy adults typically take 10–20 minutes to fall asleep. Consistently taking under 5 minutes can indicate sleep deprivation; consistently over 30 minutes may indicate insomnia. WorkItOut.Tools defaults to a 15-minute latency, adjustable in the Advanced cog.
Source: Sleep Foundation - Sleep Onset Latency
Sleep cycle A recurring progression through non-REM stages (N1, N2, N3) and REM.
Cycles average about 90 minutes but commonly range from 80 to 110 minutes, and lengthen through the night. The '5–6 cycles per night' target most sleep calculators use is a useful approximation, not a biological rule - total sleep time matters more than exact cycle boundaries.
Source: Sleep Foundation - Stages of Sleep
Chronotype An individual's natural preference for morning or evening activity.
Commonly described as lark (morning), owl (evening), hummingbird (flexible), or bear (average-tracking). Genetics and age both influence chronotype - most teenagers are natural owls, most people over 60 shift toward lark.
Source: Sleep Foundation - Chronotypes
Circadian rhythm The roughly 24-hour body clock that helps time sleep, wakefulness, hormones, and alertness.
Circadian rhythm is why the same bedtime can feel easy one week and difficult after travel, night shifts, or a schedule drift. Light exposure, meal timing, and wake time are the big practical levers for moving it.
Source: Sleep Foundation - Circadian Rhythm
Sleep debt The gap between the sleep your body needs and the sleep you actually get.
Sleep debt can build quietly from small nightly shortfalls. A plan that looks only at tonight can miss the larger pattern, which is why recovery sleep and steadier wake times matter after a demanding week.
Source: Sleep Foundation - Sleep Debt
REM sleep A sleep stage linked with vivid dreaming, memory processing, and a more active brain pattern.
REM periods tend to get longer later in the night. Cutting sleep short in the early morning can therefore trim a larger share of REM than the same number of minutes would earlier in the night.
Source: Sleep Foundation - Stages of Sleep
Deep sleep The slow-wave non-REM stage where waking up is usually hardest.
Deep sleep is concentrated more heavily in the first part of the night. This is one reason a very late bedtime can feel worse than the clock math suggests, even when total time in bed looks similar.
Source: Sleep Foundation - Stages of Sleep
Sleep inertia The groggy, slowed-down feeling that can linger after waking.
Sleep inertia is one reason cycle calculators try to avoid waking from deeper sleep. Timing helps, but total sleep, sleep debt, and wake consistency still matter more than hitting a perfect minute.
Source: Sleep Foundation - Sleep Inertia
Sleep pressure The homeostatic drive to sleep that rises the longer you are awake.
Sleep pressure is why naps can help when they are timed well but make bedtime harder when they are too long or too late. It works alongside the circadian clock, not instead of it.
Source: Sleep Foundation - Sleep Drive and Your Body Clock
Anchor sleep A consistent main sleep block kept in place when the rest of the schedule has to move.
Shift workers and frequent travellers often use anchor sleep to protect one reliable block, then add naps or smaller sleep windows around it. The goal is rhythm and enough total sleep, not a perfect one-block night.
Source: Sleep Foundation - Shift Work Sleep Tips
Light exposure The timing and brightness of light that tells your body clock when to feel alert or sleepy.
Bright morning light can help move a schedule earlier, while bright evening light can push sleep later. This is why jet lag and daylight-saving plans focus so much on light timing.
Source: Sleep Foundation - Light and Sleep
