Sleep calculator
Waking up at the end of a sleep cycle feels much better than waking up in the middle of one, even after the same number of hours. Enter the time you need to wake up and the calculator suggests bedtimes, or enter your bedtime to see the best times to set your alarm.
Uses a standard published formula, explained below. The result is an estimate, not medical advice, and its limits are explained further down the page.
Your schedule
Best bedtime
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—What a sleep cycle is
Sleep is not the same all night. You move through light sleep, deep sleep and REM sleep several times, in cycles that last about ninety minutes on average.
The cycles change as the night goes on. Deep sleep makes up most of the early cycles, and this is when most physical recovery happens. REM sleep gets longer toward the morning, which is why vivid dreams tend to come just before you wake up, and why cutting your sleep short costs you more REM sleep than deep sleep.
Waking at the end of a cycle, when you are already in light sleep, is much easier than being woken from deep sleep. Being woken from deep sleep causes sleep inertia, the heavy, confused grogginess that can last twenty to thirty minutes.
Ninety minutes is an average
Most sleep calculators present ninety minutes as exact, but it is an average. Real sleep cycles last anywhere from about 70 to 120 minutes. They differ between people, and they change during the night and with age, alcohol and stress.
The calculator also allows 14 minutes for falling asleep, which is the average for adults. If you usually take longer to fall asleep, go to bed a little earlier than the suggested times.
In practice, treat the suggested times as rough targets, not exact instructions. The useful lesson is this: seven and a half hours of sleep often feels better than eight, because eight hours can leave you waking in the middle of a cycle.
Total sleep still matters most
Timing your sleep to the end of a cycle helps, but it cannot make up for too little sleep.
Most adults need seven to nine hours a night, which is five to six cycles. Four cycles, or six hours, is not enough for the great majority of people, however well it is timed. A very small number of people genuinely function well on less sleep because of a rare gene, but far more people believe they are in that group than really are.
If you have to choose between waking in the middle of a cycle after eight hours and waking at the end of a cycle after six, choose the eight hours. Cycle timing is useful when you have some flexibility, not as a reason to sleep less.
Habits that improve sleep more than timing
Going to bed and getting up at the same time every day, including at weekends, does more for your sleep than any single night's calculation. Bright light in the morning and dim light in the evening help set the body clock that decides when you feel sleepy.
Caffeine has a half-life of about five to six hours, so an afternoon coffee is still in your system at bedtime. Alcohol helps you fall asleep faster, but it then reduces REM sleep and makes the second half of the night restless, which is why sleep after drinking does not feel refreshing.
See a doctor about ongoing sleep problems. Long-term insomnia, loud snoring with pauses in breathing, or severe daytime sleepiness despite enough hours in bed can be signs of a treatable sleep disorder. A calculator cannot diagnose these, so speak to a doctor.
Frequently asked questions
What time should I go to bed to wake up at 6 am?
Counting back in 90-minute cycles and allowing about 14 minutes to fall asleep, good bedtimes are around 8:46 pm, 10:16 pm or 11:46 pm. The first two give you six or five full cycles, which is the healthier amount for most adults.
How many hours of sleep do I need?
Most adults need seven to nine hours a night, which is five to six full sleep cycles. Teenagers usually need more. Very few people function well on less.
What is a sleep cycle?
A sleep cycle is one pass through light sleep, deep sleep and REM sleep, lasting about ninety minutes on average. Deep sleep is longest in the early cycles, and REM sleep gets longer toward the morning.
Why do I feel groggy after eight hours of sleep?
Probably because you woke up during deep sleep. Eight hours is not a whole number of ninety-minute cycles, so it often ends in the middle of one. Seven and a half or nine hours may feel better.
Is a sleep cycle always ninety minutes?
No. Ninety minutes is an average. Real cycles last roughly 70 to 120 minutes and vary between people and during the night, so treat the suggested times as approximate.
Is it better to wake at the end of a cycle or to sleep longer?
Total sleep matters more. If you must choose between waking mid-cycle after eight hours and waking at the end of a cycle after six, the eight hours are better for you.
Does alcohol help you sleep?
It helps you fall asleep faster but makes your sleep worse. It reduces REM sleep and disturbs the second half of the night, so you usually wake up feeling less rested.