Sleep Calculator

How it works

During the night you move through repeating cycles of light sleep, deep sleep and REM (dreaming) sleep. Each cycle lasts roughly 90 minutes on average, though real cycles range from about 70 to 120 minutes and change through the night.

Wake time = bedtime + time to fall asleep + (cycles × 90 minutes)

We show options for 6, 5 and 4 cycles (9, 7.5 and 6 hours). Most healthy people take 10–20 minutes to fall asleep; we use 15 minutes unless you change it.

How much sleep do adults need?

The American Academy of Sleep Medicine and Sleep Research Society recommend that adults sleep 7 or more hours per night regularly. The National Sleep Foundation recommends:

Five cycles (7.5 hours) fits these recommendations for most adults. Four cycles (6 hours) does not and is best treated as an occasional exception.

Habits that help

• Keep consistent sleep and wake times, including weekends.
• Get daylight in the morning and dim screens and lights in the evening.
• Limit caffeine after early afternoon and avoid alcohol close to bed.
• Keep your bedroom cool, dark and quiet.

Cycle timing is a guide

Research support for timing wake-ups to cycles is limited, because cycle length varies between people and nights. Getting enough total sleep, consistently, matters far more than hitting an exact cycle boundary.

The stages of a sleep cycle

Waking from deep N3 sleep is what usually causes grogginess, known as sleep inertia.

Naps

A short nap of 10–20 minutes can boost alertness without much grogginess because you're unlikely to reach deep sleep. A 90-minute nap covers a full cycle. Naps late in the afternoon can make it harder to fall asleep at night.

Sleep and health

The AASM/SRS consensus links regularly sleeping less than seven hours with weight gain, diabetes, high blood pressure, heart disease, depression, impaired immune function and a greater risk of accidents. Sleep also affects appetite hormones, which helps explain why short sleep can make weight management harder.

Choosing a realistic schedule

Pick your fixed wake-up time first, since it anchors your body clock, then use the bedtime mode to choose a lights-out time that allows five or six cycles. Keep it within about an hour on weekends.

Sleep for teens and older adults

Teenagers' body clocks naturally shift later, making early school starts difficult; 8–10 hours is recommended. Older adults often wake earlier and more often during the night, but still benefit from 7–8 hours.

Caffeine and alcohol timing

Caffeine has a half-life of roughly five hours in many adults, so an afternoon coffee can still affect sleep at bedtime. Alcohol may help you fall asleep but fragments sleep and reduces REM later in the night.

Worked examples

Example 1: Bed at 11:00 pm

11:00 + 15 min to fall asleep = 11:15. Five cycles (7.5 h) → 6:45 am; six cycles → 8:15 am; four → 5:15 am.

Example 2: Wake at 6:30 am

Six cycles back plus 15 min → 9:15 pm; five cycles → 10:45 pm; four cycles → 12:15 am.

Limitations

• Sleep cycles are not exactly 90 minutes and vary through the night.
• Doesn't account for night waking.
• Not a treatment for sleep disorders.

Who should not rely on this calculator

If you regularly feel unrefreshed despite enough sleep, snore loudly, stop breathing during sleep, have insomnia for more than a few weeks, or fall asleep during the day, talk to a doctor — these can be signs of sleep disorders such as sleep apnea. Shift workers should seek individualized advice.