Wake up between cycles, not mid-deep-sleep. Tell it when you need to wake up and it finds the bedtimes that fit whole 90-minute sleep cycles.
Go to bed at one of these times:
If you fall asleep around , set your alarm for:
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Each ~90-minute cycle ends in light sleep — the natural exit point. An alarm mid-deep-sleep is what causes that crushed-by-a-truck feeling.
7.5–9 hours of actual sleep suits most adults. Consistency matters more than perfection — same time daily beats heroic weekends.
Pair a fixed bedtime with white noise and a dim screen — your falling-asleep buffer will shrink on its own.
Sleep proceeds through cycles of light sleep, deep slow-wave sleep and REM sleep. A cycle averages about 90 minutes, which is where the familiar advice to sleep in 90-minute multiples comes from — waking at the end of a cycle, in light sleep, generally feels better than waking from deep sleep.
The 90-minute figure is a population average, not a personal constant. Real cycles range from roughly 70 to 120 minutes, vary between people, and vary within a single night for the same person. Cycle composition also shifts through the night: deep sleep dominates early cycles and REM lengthens toward morning, which is why losing the last two hours of sleep costs disproportionately more REM.
So a cycle-based bedtime calculation is a reasonable heuristic and not a precise instrument. If waking at a calculated time still feels bad, your cycle length probably differs from the assumed one — adjusting the target by 15 or 20 minutes either way is a sensible experiment.
The American Academy of Sleep Medicine and the Sleep Research Society recommend 7 or more hours per night for adults, with 7 to 9 the commonly cited range. Teenagers need 8 to 10, school-age children 9 to 12, and infants considerably more.
Counting from the moment you actually fall asleep, not the moment you get into bed. Most people need 10–20 minutes to drop off, so subtract that from the figure below.
| Asleep at | Wake at | Total sleep | Full cycles |
|---|---|---|---|
| 9:00 pm | 5:00 am | 8 hours | 5 cycles |
| 9:00 pm | 6:00 am | 9 hours | 6 cycles |
| 10:00 pm | 5:00 am | 7 hours | 4–5 cycles |
| 10:00 pm | 6:00 am | 8 hours | 5 cycles |
| 10:00 pm | 7:00 am | 9 hours | 6 cycles |
| 11:00 pm | 5:00 am | 6 hours | 4 cycles |
| 11:00 pm | 6:00 am | 7 hours | 4–5 cycles |
| 11:00 pm | 7:00 am | 8 hours | 5 cycles |
| 11:00 pm | 8:00 am | 9 hours | 6 cycles |
| 12:00 am | 6:00 am | 6 hours | 4 cycles |
| 12:00 am | 7:00 am | 7 hours | 4–5 cycles |
| 12:00 am | 8:00 am | 8 hours | 5 cycles |
| 1:00 am | 7:00 am | 6 hours | 4 cycles |
| 1:00 am | 9:00 am | 8 hours | 5 cycles |
The population of genuine short sleepers — people who function normally on 6 hours or less due to identified genetic variants — is very small, well under 1 percent, and far smaller than the number of people who believe they belong to it. Research on sleep restriction consistently finds that performance degrades measurably while subjective sleepiness plateaus, so people become impaired without feeling proportionally worse. Self-assessment is not reliable here.
Sleep debt accumulates and is only partly repayable. A weekend lie-in recovers some of it, but studies of recovery sleep find that metabolic and attentional measures do not fully return after a single weekend, and irregular timing carries its own cost. Consistency of schedule matters roughly as much as duration.
Regularity is the strongest single lever: going to bed and waking at consistent times, including at weekends, stabilises circadian timing more effectively than any individual habit. Morning daylight exposure is the most powerful signal for anchoring that rhythm.
Temperature matters more than most people expect. Core body temperature must fall for sleep to begin, so a cool bedroom — commonly cited around 16 to 19 °C — helps, and a warm bath an hour or two before bed helps by promoting heat loss afterwards.
Caffeine has a half-life of about five to six hours, so an afternoon coffee is still substantially present at bedtime, and it reduces deep sleep even when it does not prevent sleep onset. Alcohol shortens sleep latency and then fragments the second half of the night and suppresses REM. Persistent difficulty sleeping, loud snoring with pauses in breathing, or daytime sleepiness despite adequate time in bed are reasons to see a doctor — insomnia and sleep apnoea are common, treatable, and not addressed by a bedtime calculator. This tool does arithmetic on cycle lengths and is not medical advice.
No — real cycles run roughly 80–110 minutes and lengthen through the night. 90 is the practical average; treat the suggested times as targets, not prescriptions.
Pick the closest one and keep it consistent. A regular "good enough" schedule beats an optimal one you can't keep.
Short naps (20 min) avoid deep sleep entirely; full-cycle naps (90 min) wake you at a cycle boundary. The in-between zone (45–60 min) is where nap grogginess lives.
It is an average. Real cycles run roughly 70 to 120 minutes and vary between people and across a single night, so cycle-based bedtimes are a useful heuristic rather than a precise calculation.
Partly. Recovery sleep restores some function, but studies find metabolic and attention measures do not fully normalise after one weekend, and irregular timing carries its own cost. Consistency matters about as much as total duration.
Eight hours, which is about five full 90-minute sleep cycles. Because most people take 10 to 20 minutes to fall asleep, going to bed at 11pm usually means closer to 7 hours 40 minutes of actual sleep.
Eight hours, roughly five sleep cycles. From 11pm to 6am is seven hours, and from midnight to 6am is six.
Work backwards from when you must wake. Most adults need seven to nine hours, so subtract that plus about 15 minutes to fall asleep. Waking at the end of a cycle rather than mid-cycle tends to feel easier, which is what the calculator above targets.