What time should I go to bed? Sleep cycles and a simple calculator

To wake up feeling as rested as you can, plan your sleep in full cycles of about 90 minutes, and allow time to fall asleep. Count back from the time you need to get up in multiples of 90 minutes: five cycles is 7.5 hours and six is 9 hours. Then subtract the time you usually take to fall asleep. Cycle timing only helps when the total is already enough, and for most adults that means at least seven hours.

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What a sleep cycle is

Sleep moves through stages. N1 is the lightest stage, when you first drift off. N2 is light sleep and makes up much of the night. N3 is deep, slow-wave sleep. REM is the stage linked to dreaming. The Sleep Foundation describes a complete cycle as taking about 90 minutes. A typical cycle moves from light sleep into deep sleep, back through light sleep, and then into REM.

Deep sleep is most common in the early cycles, and REM periods get longer toward morning. The 90-minute figure is an average. Individual cycles vary, and one common summary gives a range of roughly 70 to 110 minutes. Treat the times the calculator produces as a guide, not a guarantee.

How much sleep adults need

The U.S. Centers for Disease Control and Prevention (CDC) advises that adults aged 18 to 60 get seven or more hours of sleep a night on a regular basis. The CDC's guidance follows the 2015 consensus statement from the American Academy of Sleep Medicine and the Sleep Research Society. Adults aged 61 to 64 are listed at 7 to 9 hours, and adults 65 and older at 7 to 8 hours.

Seven hours is a minimum for most adults, not a target to aim just above. Regularly sleeping less is linked with poorer health over time.

A worked example

Suppose you have to get up at 6:30 in the morning. Count back, then add 15 minutes for falling asleep:

Sleep cycles	Total sleep	Get into bed by
4	6 hours	00:15
5	7.5 hours	22:45
6	9 hours	21:15

Five cycles, at 22:45, is the first option that meets the seven-hour guidance. Six cycles gives a longer night if you can fit it in.

You can check the sum by hand. 6:30 minus 7.5 hours is 23:00, and 23:00 minus 15 minutes is 22:45. A calculator just does this for every option at once.

Why waking mid-cycle feels groggy

Waking from deep sleep often leaves you foggy for a while. Researchers call this sleep inertia. The Sleep Foundation says it generally lasts 15 to 60 minutes. Waking from deep sleep tends to cause more inertia than waking from light sleep or REM, and sleep debt makes it last longer.

Timing your alarm to the end of a cycle can help, because you are more likely to be in light sleep. The timing is an average, though, and no alarm can guarantee that you are in light sleep. Build in a few minutes after the alarm before you judge how rested you feel.

Allow for falling asleep

The calculator asks how long you take to fall asleep, and that number changes the answer. If you do not know, note the time you turn off the light on a few nights and the time you actually feel asleep. Use your own figure rather than a default.

Keep the wake-up time steady

Your body clock works best with a regular wake-up time. A large change in when you get up, such as a long lie-in on the weekend, shifts that clock, so a steady alarm often does more than a perfect bedtime. Choose a wake time you can keep most days, then plan the bedtime backwards from it.

Using the online timer as a bedtime countdown can help you start winding down at the right moment. For why a timer in a background tab may run late, see countdown timers in background tabs.

When to see a doctor

Ask a doctor for help if you regularly struggle to fall asleep or stay asleep, if you snore loudly with pauses in breathing, if you feel very sleepy during the day, especially when driving or working, or if you are still tired after getting enough sleep.

Quick answers

Can I function well on six hours? Some people try, but the guidance for most adults is seven or more hours on a regular basis.

Do I need exactly 90 minutes per cycle? No. The figure is an average, so the calculator gives a range of sensible options rather than a precise promise.

Is it better to wake in light sleep? Often it feels easier, but you cannot control exactly when you will be in light sleep. A steady wake time matters more.

Do naps follow cycles too? Keeping a nap to 20 to 30 minutes is the usual advice, because a short nap is less likely to run into deep sleep, where waking causes the most grogginess.

This is general information, not medical advice. See a doctor about persistent sleep problems or daytime sleepiness that affects your safety.