How to Calculate the Best Time to Sleep
The trick to waking up refreshed is not just sleeping longer — it is waking up at the right moment. Sleep runs in cycles of about 90 minutes, and waking at the end of a cycle feels much easier than waking in the middle of deep sleep. This guide shows how to calculate your best bedtime and wake-up time from those cycles, with worked examples you can check, and the habits that make the maths actually work.
How to calculate the best time to sleep — step by step
Know your wake-up time
Start with the time you must wake up — for example, 7:00 AM for work or school. That is the fixed point; every bedtime is counted back from it.
Count back in 90-minute cycles
Subtract 90 minutes at a time from your wake-up time. From 7:00 AM: one cycle back is 5:30 AM, two cycles 4:00 AM, three cycles 2:30 AM, four cycles 1:00 AM, five cycles 11:30 PM and six cycles 10:00 PM.
Pick 5 or 6 cycles (7.5 or 9 hours)
Most adults need 7 to 9 hours — 5 cycles (7.5 hours) or 6 cycles (9 hours). For a 7:00 AM wake-up, that means 11:30 PM or 10:00 PM. The 4-cycle (6-hour) and 3-cycle (4.5-hour) options are there for short nights, not for every night.
Add about 15 minutes to fall asleep
The bedtime the calculator shows is the time to be asleep, not the time to get into bed. Most people take about 15 minutes to fall asleep, so be in bed by 11:15 PM if you want to be asleep by 11:30 PM.
Work forward when you sleep now
If you are going to bed now, add 15 minutes for falling asleep, then add 90-minute cycles: sleeping at 11:00 PM means waking at 5:15 AM (4 cycles), 6:45 AM (5 cycles) or 8:15 AM (6 cycles). Pick the wake-up time that fits your morning.
Keep the same times every day
A regular bedtime and wake-up time — even at weekends — trains your body clock, so you fall asleep faster and wake up at the end of a cycle more often. The calculator only helps if the times you pick are times you actually keep.
Sleep cycle examples — wake at 7:00 AM
| What you want | How to work it out | Example |
|---|---|---|
| 6 cycles — 9 hours | wake time minus 6 x 90 minutes | 7:00 AM - 9 hours = 10:00 PM (recommended) |
| 5 cycles — 7.5 hours | wake time minus 5 x 90 minutes | 7:00 AM - 7.5 hours = 11:30 PM (recommended) |
| 4 cycles — 6 hours | wake time minus 4 x 90 minutes | 7:00 AM - 6 hours = 1:00 AM |
| 3 cycles — 4.5 hours | wake time minus 3 x 90 minutes | 7:00 AM - 4.5 hours = 2:30 AM |
| Sleep now (11:00 PM) | sleep time + 15 min + cycles x 90 min | 4 cycles = 5:15 AM, 5 cycles = 6:45 AM, 6 cycles = 8:15 AM |
| Sleep duration | wake time minus bedtime | 11:00 PM to 7:00 AM = 8 hours = about 5.33 cycles |
Sleep cycles average about 90 minutes and vary from person to person — these are estimates, not exact science and not medical advice.
Five tips for better sleep
- Aim for 5 or 6 cycles: 7.5 hours (5 cycles) is the sweet spot for most adults; 9 hours (6 cycles) is best when you are tired, ill or catching up on sleep. Less than 6 hours every night harms your health over time.
- Get into bed 15 minutes early: The calculator's bedtime is the time to be asleep. Allow about 15 minutes to fall asleep — being in bed early is the easiest way to hit your cycle.
- Keep the same wake-up time: Waking at the same time every day, even at weekends, steadies your body clock far more than going to bed at the same time. Pick a wake-up time you can keep.
- Dim the lights before bed: Bright light, especially from phones and screens, tells your brain it is daytime. Dim the lights and put screens away for the last 30 minutes before bed.
- Avoid caffeine late in the day: Caffeine can stay in your body for 6 hours or more. A coffee at 4:00 PM can still be keeping you awake at 10:00 PM — switch to water or herbal tea in the afternoon.
Frequently asked questions
What is the best time to go to bed?
It depends on your wake-up time. Count back 5 or 6 sleep cycles of 90 minutes from when you must wake up: for a 7:00 AM wake-up, go to bed at 11:30 PM (7.5 hours) or 10:00 PM (9 hours). Use the bedtime calculator on the home page for your own wake-up time.
How many sleep cycles do I need?
Most adults need 5 to 6 full cycles a night — 7.5 to 9 hours. Children and teenagers need more. Fewer than 4 cycles (6 hours) every night is not enough for most people.
Is 6 hours of sleep enough?
6 hours is 4 sleep cycles, and it is the minimum for a short night — but most adults need 7 to 9 hours. One 6-hour night is fine now and then; every night is not enough, and you will feel it.
How long is one sleep cycle?
About 90 minutes on average — light sleep, then deep sleep, then dream (REM) sleep. Real cycles vary from about 70 to 110 minutes and change through the night, so 90 minutes is a useful average, not an exact number.
Does this site store my sleep times?
No. Every calculation happens in your browser and your times are never sent to any server or stored anywhere.