Some mornings you bounce out of bed after six hours. Other mornings you get eight and feel like you've been hit by a truck. The difference often isn't how long you slept — it's when in your sleep cycle the alarm went off.
How sleep cycles work
Sleep isn't one long, flat state. Through the night, your brain moves through repeating cycles, each lasting roughly 90 minutes:
- Light sleep — you drift off and are easy to wake.
- Deep sleep — your body repairs itself; waking here feels awful.
- REM sleep — dreaming, memory processing, and lighter brain activity.
At the end of each cycle, sleep becomes lighter for a short time. Wake up then, and you feel fairly alert. Wake up in the middle of deep sleep, and you get sleep inertia — that heavy, foggy feeling that can last for 30 minutes or more.
The simple math
To wake up between cycles, count backward from your alarm in 90-minute blocks, plus about 15 minutes to fall asleep.
Say you need to be up at 7:00 AM:
- 6 cycles (9 hours): bed at 9:45 PM
- 5 cycles (7.5 hours): bed at 11:15 PM
- 4 cycles (6 hours): bed at 12:45 AM
Notice that going to bed at 11:15 PM might leave you feeling better than going to bed at 10:30 PM — even though it's less total sleep — because the alarm lands at the end of a cycle.
How much sleep do you actually need?
Most adults do best with 7–9 hours a night, which works out to 5 or 6 cycles. Four cycles is fine for the odd short night, but making it a habit builds up sleep debt that no clever timing can fix.
How to use it
The Sleep Calculator works in both directions. Choose "I want to wake up at" to get the best bedtimes, or "I'm going to bed at" to get the best alarm times. Heading to bed right now? Tap "Going to bed now" and it uses the current time. Five cycles (7.5 hours) is highlighted as a good target, with 6, 4, and 3 cycles listed too.
A few habits that make the numbers work even better:
- Keep a consistent wake-up time, even on weekends. It anchors your body clock.
- Adjust by 10–15 minutes. Cycles vary from person to person, so tweak until mornings feel easy.
- Dim screens before bed. Bright light late at night can delay falling asleep.
Travelling across time zones? The Time Zone Converter helps you figure out what time it will be at home, and the Countdown Timer is handy for a wind-down routine before lights out.
This calculator is a general guide, not medical advice. If you regularly struggle to fall asleep or stay awake during the day, talk to a doctor.