Sleep Calculator

Count back from your wake-up time in 90-minute sleep cycles and add about 15 minutes to fall asleep. For a 7:00 AM alarm, that means 9:45 PM for six cycles (9 hours), 11:15 PM for five (7.5 hours) or 12:45 AM for four (6 hours). Adults need 7 or more hours, so five or six cycles is the target.

The calculator above does that sum for any time you choose. The bedtime chart under it prints the answer for every wake-up time from 5:00 to 9:00 AM, so you can look yours up at a glance.

The tool has three tabs.

1. Wake up at… Enter the time your alarm goes off. You get three bedtimes: six, five and four full cycles before it, plus the time it takes you to fall asleep. 2. Going to bed… Start from now, or any time you plan to turn the light off, and get three wake-up times that land between cycles. 3. Sleep debt. Add the hours you actually slept over the last seven nights, how well you slept and how the next day felt. The tab adds up the shortfall against your need and suggests a gentle plan to make it up.

The settings let you change the minutes it takes you to fall asleep (5 to 30) and the length of a cycle (80, 90 or 100 minutes). If you usually lie awake for half an hour, set it; the bedtimes move to match.

Through the night you move between lighter and deeper non-REM sleep and REM sleep, the stage with most dreaming, in repeating cycles. The National Heart, Lung, and Blood Institute puts a cycle at 80 to 100 minutes, with four to six cycles a night. The Institute of Medicine describes the first cycle as 70 to 100 minutes and later ones as roughly 90 to 120. Deep sleep comes mostly early in the night and REM mostly later.

So 90 minutes is a handy average, not a rule, and no calculator knows your exact cycle. Timing your alarm between cycles is a reasonable guess that many people find makes waking easier. If you often wake groggy at a “cycle” time, try the 80 or 100-minute setting for a week and see which suits you. Getting enough total sleep matters far more than hitting a cycle boundary.

The American Academy of Sleep Medicine and the Sleep Research Society agree that adults should sleep 7 or more hours a night on a regular basis, and that regularly sleeping less is linked with weight gain, high blood pressure, heart disease, low mood and other problems (Watson and colleagues, 2015). They add that more than 9 hours may be right for young adults, for people recovering from sleep debt and for people who are ill.

Within that, needs differ. A good way to find yours is the number of hours after which you wake rested without an alarm, on a holiday week once the first catch-up nights are over. The sleep debt tab lets you set your need anywhere from 7 to 9 hours.

Sleep debt is the gap between the sleep you need and the sleep you get, added up over days. It is easy to miss. In a lab study by Van Dongen and colleagues (2003), people kept to 6 hours a night for two weeks did as badly on attention tests as people kept awake for up to two nights in a row, yet rated themselves only slightly sleepier.

A big lie-in does less than it feels like. In a 2019 study by Depner and colleagues, people who slept in at the weekend after short weeknights still showed worse insulin sensitivity once the short nights began again. The calculator’s plan works the other way:

1. Go to bed earlier and keep your usual alarm, so your body clock does not slide later. 2. Add no more than about an hour a night, spread over several nights. 3. On a hard day, take a short nap. NIOSH advises people on daytime schedules to keep naps under 20 minutes: it lifts alertness for a couple of hours without disrupting the night. 4. If the debt keeps coming back, look at what is eating your evenings.

Our guide to sleep debt goes into more detail, and if bedtime keeps slipping because the late evening is the only time that feels like your own, read about revenge bedtime procrastination.

The sleep debt tab asks how each next day felt for a reason. In a six-week study of 208 adults who logged sleep and mood on their phones, a night’s sleep quality shaped the next day’s mood more than the day’s mood shaped the following night (Triantafillou and colleagues, 2019). An older review of 19 studies found that mood suffers from sleep loss even more than thinking or reaction speed does (Pilcher and Huffcutt, 1996).

After a few nights, the tab shows your own version: how the days felt after nights you got your hours compared with short ones. For more on why, see how sleep affects mood. To keep sleep next to mood, energy and stress every day, use the 60-second mental health check-in, and to see how your nights show up in your body, try HRV by age.

Knowing your bedtime does not help much if your mind starts racing the moment you lie down. A few things that help many people: write tomorrow’s worries down before bed with a brain dump, try 4-7-8 breathing or a sleep meditation for anxiety, and read our guide on how to stop overthinking at night. If you lie awake even when your mind is quiet, how to fall asleep faster covers the wind-down, the 20-minute rule and when to see a doctor.

The calculator works with averages and with the hours you type in. It cannot measure your sleep stages or tell you why you sleep badly. If you are often exhausted despite enough time in bed, snore loudly with pauses in your breathing, or have struggled to fall or stay asleep most nights for weeks, talk to a doctor: those are worth checking properly.