Why can six hours feel better than seven?

Because six is four complete cycles and seven leaves you mid-cycle. Waking from deep sleep produces grogginess that lasts, which is sleep inertia.

Should I sacrifice sleep to hit a cycle?

No. Total sleep matters more. Use the cycles to choose between two bedtimes that are close, not to justify an hour less.

Health & Fitnesslifestyle

Sleep Cycle Calculator

Bedtimes that let you wake at the end of a sleep cycle rather than the middle of one. Waking mid-cycle from deep sleep is what produces grogginess even after eight hours.

Also called: bedtime calculator, when should i sleep.

Best times to go to bed

21:15, 22:45, 00:15

To wake at 06:30 feeling rested, go to bed at one of: 21:15, 22:45, 00:15. Those give 6, 5 or 4 complete cycles of 90 minutes, allowing 15 minutes to fall asleep.

Cycles those give

6, 5 or 4

For six cycles (about 9 hours)

21:15

For five cycles (about 7.5 hours)

22:45

For four cycles (about 6 hours)

00:15

Waking at

06:30

How this is calculated

For reference only. Verify against your clinical protocol before acting on this number.

Method and background

This is what the calculation gives for the numbers you entered. It is an estimate, not advice, and it knows nothing about your situation beyond those numbers. Rules for United States change on a published schedule; the effective date is shown on every rule-based tool.

How this is calculated

Sleep runs in cycles of roughly ninety minutes, moving from light through deep to REM sleep. Waking at the end of a cycle, in light sleep, feels far better than waking in the middle of deep sleep, which is why six hours can feel better than seven. Working backwards from the wake time in whole cycles, and adding the time it takes to fall asleep, gives the bedtimes worth aiming at.

bedtime = wake time, less a whole number of cycles, less the time it takes to fall asleep

w: Wake time
n: Number of cycles
c: Cycle length
f: Time to fall asleep

Worked examples

Each of these is asserted on every build. If a change to the engine ever moved one of these answers, the build would fail before the page could print it.

waking at half six

Wake at hour: 6
Wake at minute: 30
Cycle length: 90 minutes
Time to fall asleep: 15

Best times to go to bed21:15, 22:45, 00:15

6:30 less 7.5 hours less 15 minutes

Open this example

no time to fall asleep

Wake at hour: 6
Wake at minute: 30
Cycle length: 90 minutes
Time to fall asleep: 0

Best times to go to bed21:30, 23:00, 00:30

boundary

Open this example

a longer cycle shifts everything earlier

Wake at hour: 6
Wake at minute: 30
Cycle length: 120 minutes
Time to fall asleep: 15

Best times to go to bed18:15, 20:15, 22:15

degenerate case

Open this example

Method and limits

What it assumes

A ninety minute cycle, which is an average. Individual cycles range from about seventy to a hundred and twenty minutes.

What it deliberately does not model

Cycle length varies between people and across a night, so these are targets rather than precision.
Total sleep matters more than cycle alignment. Five cycles beats four regardless of timing.

Formula version 1.0.0 · definition 1.0.0 · United States · Report a problem with this calculator

Frequently asked questions

Why can six hours feel better than seven?: Because six is four complete cycles and seven leaves you mid-cycle. Waking from deep sleep produces grogginess that lasts, which is sleep inertia.
Should I sacrifice sleep to hit a cycle?: No. Total sleep matters more. Use the cycles to choose between two bedtimes that are close, not to justify an hour less.