How does it work?
Sleep runs in cycles of about 90 minutes: light sleep, then deep sleep, then dream (REM) sleep. If your alarm goes off during deep sleep, you wake up groggy even after a long night. Waking at the end of a cycle leaves you feeling lighter and more alert.
How much sleep do I need?
| Age | Recommended hours |
|---|---|
| Children (6–13) | 9–11 hours |
| Teenagers (14–17) | 8–10 hours |
| Adults (18–64) | 7–9 hours |
| Older adults (65+) | 7–8 hours |
Per the National Sleep Foundation. That’s why the calculator recommends 5 or 6 cycles (7.5 or 9 hours).
Example
To wake at 7:00 AM if you take about 15 minutes to fall asleep: go to bed at 9:45 PM for 6 cycles (9 hours), 11:15 PM for 5 cycles (7½ hours) or 12:45 AM for 4 cycles (6 hours). Waking at the end of a cycle rather than in the middle helps you feel less groggy.
Tips for better sleep
- Keep the same bedtime and wake-up time, even on days off.
- Cut down on screens before bed and on caffeine after mid-afternoon.
- Keep the bedroom dark, quiet and at a comfortable temperature.
Frequently Asked Questions
Why do I wake up tired after 8 hours?
Your alarm may be going off mid-cycle in deep sleep. Try sleeping or waking 15–30 minutes earlier or later so you wake at the end of a cycle. Sleep quality matters too: screens before bed and late caffeine reduce it.
What is the best nap length?
A short 20-minute nap refreshes without entering deep sleep, or 90 minutes for a full cycle. Anything in between can leave you groggy.
Is a sleep cycle always 90 minutes?
No — 90 minutes is an average; cycles range from about 70 to 120 minutes depending on the person and the night. Use the times as a guide, and if you still wake up groggy, try 15 minutes earlier or later.
Can I catch up on sleep at the weekend?
It helps a little, but regular, sufficient sleep is much better than short nights all week and long ones at the weekend.
How should I sleep during Ramadan?
Try splitting your sleep: one block after taraweeh until suhoor, and another after Fajr. Use the calculator to pick wake-up times at the end of a cycle.