Sleep cycles explained
Each sleep cycle is approximately 90 minutes and includes light sleep, deep sleep, and REM sleep. Waking at cycle end minimizes grogginess.
Health tools
Sleep Calculator
Find your ideal bedtime or wake time based on sleep cycles.
Complete guide
How to use Sleep Calculator
Sleep occurs in 90-minute cycles. Waking at the end of a cycle (rather than the middle) leaves you feeling more refreshed. The Sleep Calculator suggests times that align with completing 4, 5, or 6 full cycles.
Choose whether you want to know when to wake up (given a bedtime) or when to go to sleep (given a wake time). The calculator adds a 14-minute allowance for falling asleep.
How many cycles do you need?
Most adults need 5–6 complete cycles (7.5–9 hours). Teenagers need slightly more. Older adults sometimes function well on 4–5.
The 14-minute allowance
The calculator assumes you fall asleep 14 minutes after lying down — the average for most adults.
Answers
Sleep Calculator FAQs
What is a sleep cycle?
A sleep cycle is a 90-minute period that moves through light sleep, deep sleep, and REM sleep. Most adults complete 5–6 per night.
Why do I feel groggy even after 8 hours?
You may have woken during deep sleep. Waking at the end of a 90-minute cycle results in less grogginess.
How many hours of sleep do I need?
Most adults need 7–9 hours. This equates to 5–6 complete 90-minute sleep cycles.
What time should I wake up?
Enter your desired bedtime and this calculator shows optimal wake times based on full sleep cycle completion.
What time should I go to sleep?
Enter your desired wake time and this calculator shows when you should go to bed for 4, 5, or 6 complete cycles.
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