Sleep Calculator

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Work out the best time to wake up or go to bed so you rise between sleep cycles, not in the middle of one. Based on 90-minute cycles — everything runs in your browser. Free on LK Forge — no sign-up.

Why sleep cycles matter

Sleep runs in cycles of roughly 90 minutes, moving through light sleep, deep sleep and REM before starting over. Deep sleep dominates the earlier cycles of the night, while REM stretches longer as morning approaches. If an alarm interrupts you in the middle of a deep-sleep stage, you tend to wake up groggy and disoriented — an effect called sleep inertia — even if you technically got enough total hours. Waking up at the boundary between two cycles, when your body is already in a lighter stage, generally feels far easier.

How this calculator works

It assumes about 15 minutes to fall asleep, then counts back or forward in whole 90-minute cycles from the time you enter. In "When to wake up" mode, give it a bedtime and it lists wake-up times that land at cycle boundaries. In "When to go to bed" mode, give it a wake-up time and it lists bedtimes that work backward the same way. For most adults, five to six full cycles — about 7.5 to 9 hours of sleep — covers the recommended range, so those options are marked Recommended.

Recommended sleep by age

Age group	Recommended sleep
Newborn (0–3 mo)	14–17 hours
Infant (4–11 mo)	12–16 hours
Toddler (1–2 yr)	11–14 hours
Preschool (3–5 yr)	10–13 hours
School age (6–13 yr)	9–12 hours
Teen (14–17 yr)	8–10 hours
Adult (18–64 yr)	7–9 hours
Older adult (65+ yr)	7–8 hours

Ranges from the National Sleep Foundation.

Frequently Asked Questions

How many hours of sleep do I need by age?

Sleep needs fall with age. The National Sleep Foundation recommends 14–17 hours for newborns (0–3 months), 12–16 hours for infants (4–11 months), 11–14 hours for toddlers (1–2 years), 10–13 hours for preschoolers (3–5 years), 9–12 hours for school-age children (6–13 years), 8–10 hours for teenagers (14–17 years), 7–9 hours for adults (18–64 years), and 7–8 hours for older adults (65+). See the full table above. These are ranges rather than targets: inside a band, some people are fine at the low end while others need the top of it. This calculator works to the adult 7-to-9-hour range, where five 90-minute cycles is about 7.5 hours of sleep and six cycles about 9.

How does the calculator decide which wake-up times to recommend?

The sleep calculator picks wake-up times by counting forward from your bedtime in whole 90-minute cycles, after adding 15 minutes for the time it typically takes to fall asleep. The six-cycle and five-cycle options (about 9 and 7.5 hours of sleep) are marked Recommended because they fall inside the range most adults need, while four and three cycles are shown as shorter alternatives. It runs the same arithmetic in reverse when you switch to bedtime mode: from a wake-up time it subtracts whole cycles plus the 15-minute buffer to give the times you should be getting into bed. Both directions use a flat 90-minute cycle length rather than trying to model how cycle length drifts through the night.

Why 15 minutes to fall asleep?

Fifteen minutes is a commonly used average for how long it takes a person to drift from lying down to actually being asleep, sometimes called sleep latency. The calculator adds this buffer before counting cycles so the suggested times reflect when you get out of bed, not just when you close your eyes. Real sleep latency varies a lot: consistently dropping off in under five minutes is a sign of sleep debt, while routinely taking more than 30 minutes is one of the markers used for insomnia. If you know you are usually slower than 15 minutes, shift the suggested bedtime earlier by the difference.

Is it better to wake up after 5 cycles or 6 cycles?

Both five and six sleep cycles are reasonable and are marked Recommended here. Five cycles (about 7.5 hours) suits people who feel rested on the shorter end of the healthy adult range, while six cycles (about 9 hours) suits people who need more. Waking at the end of either cycle, rather than in the middle of one, is what tends to reduce grogginess.

What if I can't hit one of the suggested times exactly?

Treat the times as a guide rather than a strict schedule. Falling asleep a little earlier or later than planned, or waking a few minutes off from a suggested time, will not undo the benefit of aiming for a full number of cycles instead of an arbitrary hour count. A cycle is 90 minutes long, so being 10 or 15 minutes out still leaves you near a boundary rather than deep in the middle of one. Consistency matters more than precision: keeping your bedtime within about an hour of the same point every night does more for how you feel than hitting any single suggested time exactly.

Is 6 hours of sleep enough?

For most adults, six hours of sleep is not enough. Six hours is four full 90-minute cycles, so you wake at a cycle boundary and may feel less groggy than at an odd hour, but it still falls below the 7 to 9 hours the National Sleep Foundation recommends for adults. Regularly sleeping under 7 hours is linked to worse concentration, mood and health. A few people feel rested on the short end, but 6 hours is best treated as an occasional minimum rather than a nightly target.

Sources

Based on ~90-minute sleep cycles (Carskadon & Dement, sleep-cycle physiology). Cycle length varies roughly 70–120 minutes between people and across the night, so wake times are approximate.

These results are estimates for general fitness and educational use, not medical advice. Individual needs vary — consult a qualified healthcare or fitness professional before making significant diet, training, or health decisions.

Sleep Calculator

Why sleep cycles matter

How this calculator works

Recommended sleep by age

Frequently Asked Questions

Sources

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