Sleep Calculator (bedtime & wake-up time in 90-minute cycles)

Understanding the sleep calculator — concept and principle

This calculator suggests bedtimes for a chosen wake-up time, or wake-up times for a chosen bedtime, so that you wake at the end of a sleep cycle. It adds the time you usually take to fall asleep and counts in whole cycles of about 90 minutes.

How it is calculated

Bedtime = wake-up time − time to fall asleep − n × cycle length

Wake-up time = bedtime + time to fall asleep + n × cycle length

Sleep time = n × cycle length; time in bed = sleep time + time to fall asleep

• n = 4, 5 or 6 cycles; the default cycle length is 90 minutes.
• "−1 d" means the evening before.

The principle

Sleep alternates between light, deep and REM sleep in cycles that average about 90 minutes, though they range roughly from 70 to 120 minutes. Waking from light sleep at the end of a cycle usually feels easier than being woken from deep sleep. Counting back in whole cycles gives bedtimes that line up with those natural wake points. Five cycles give 7.5 hours of sleep, which meets the adult recommendation of 7 hours or more.

Worked example

Default inputs: wake at 07:00, 15 minutes to fall asleep, 90-minute cycles.

1. Five cycles = 5 × 90 = 7 h 30 min of sleep.
2. Bedtime = 07:00 − 7 h 30 min − 15 min = 23:15 the evening before; time in bed 7 h 45 min.
3. Six cycles (9 h 00 min) → 21:45; four cycles (6 h 00 min, a short night) → 00:45.

How to read the result

Choose the bedtime that gives you at least 7 hours of sleep. Cycle length varies between people and through the night, so treat the times as a guide; a regular schedule matters more than hitting an exact minute. If you often need an alarm to wake or feel sleepy in the day, you may need more sleep.

Common mistakes

• Forgetting the time it takes to fall asleep.
• Choosing four cycles regularly; 6 hours is short for most adults.
• Expecting the 90-minute rule to be exact for everyone.

Frequently asked questions

How many sleep cycles do adults need?

Usually about five, giving roughly 7.5 hours, but needs vary.

Why do I wake up tired after 8 hours?

You may have woken during deep sleep, slept poorly, or have a sleep disorder.

Do naps follow the same rule?

Short naps of about 20 minutes avoid deep sleep; a 90-minute nap covers one full cycle.

Sources: Watson NF, Badr MS, Belenky G, et al., "Recommended amount of sleep for a healthy adult: a joint consensus statement of the AASM and SRS", Sleep 2015;38(6):843–844 · Carskadon MA, Dement WC, "Normal human sleep: an overview", in Principles and Practice of Sleep Medicine