To get 8 hours of sleep, go to bed 8 hours and 15 minutes before your alarm. For a 6:30 AM wake-up, that's 10:15 PM; for 7:00 AM, it's 10:45 PM. The extra 15 minutes covers the time it usually takes to fall asleep, so you get 8 hours of actual sleep rather than 8 hours in bed.
The chart below does the math for every wake-up time from 5:00 to 9:00 AM. For any other time, the sleep calculator works it out, or enter a bedtime and alarm into the hours of sleep calculator to see exactly how much sleep you'll get.
Free toolSleep CalculatorBest bedtime or wake-up time, based on 90-minute sleep cycles.Sleep calculator for 8 hours: the formula
Bedtime = wake-up time minus 15 minutes minus the hours of sleep you want. For 8 hours, that means subtracting 8 hours 15 minutes.
Worked example for a 6:30 AM alarm:
- Start with the wake-up time: 6:30 AM.
- Subtract 8 hours of sleep: 10:30 PM.
- Subtract 15 minutes to fall asleep: 10:15 PM in bed.
For 7 hours, subtract 7 hours 15 minutes. For 9 hours, subtract 9 hours 15 minutes. If you usually take 30 minutes to drift off, subtract 30 instead of 15.
Bedtime chart for 7, 8 and 9 hours of sleep
Find your wake-up time and read across. Every bedtime includes 15 minutes to fall asleep.
| Wake-up time | 9 h of sleep | 8 h of sleep | 7 h of sleep |
|---|---|---|---|
| 5:00 AM | 7:45 PM | 8:45 PM | 9:45 PM |
| 5:30 AM | 8:15 PM | 9:15 PM | 10:15 PM |
| 6:00 AM | 8:45 PM | 9:45 PM | 10:45 PM |
| 6:30 AM | 9:15 PM | 10:15 PM | 11:15 PM |
| 7:00 AM | 9:45 PM | 10:45 PM | 11:45 PM |
| 7:30 AM | 10:15 PM | 11:15 PM | 12:15 AM |
| 8:00 AM | 10:45 PM | 11:45 PM | 12:45 AM |
| 8:30 AM | 11:15 PM | 12:15 AM | 1:15 AM |
| 9:00 AM | 11:45 PM | 12:45 AM | 1:45 AM |
The 7-hour column is the minimum for adults, not the target. A panel from the American Academy of Sleep Medicine and Sleep Research Society recommends adults regularly sleep at least 7 hours, and many people feel best closer to 8. If you're unsure which you need, 7 vs 8 hours of sleep compares the two.
Is 8 hours of sleep the same as 8 hours in bed?
No. Eight hours in bed usually gives you less than 8 hours of sleep. You need time to fall asleep, and most people wake briefly a few times during the night.
If you get into bed at 10:30 PM and your alarm goes at 6:30 AM, you've spent 8 hours in bed but slept about 7 hours 45 minutes at most. Add a couple of short wake-ups and it may be closer to 7.5. Time in bed vs time asleep explains how to measure the difference.
That's why this chart counts back from actual sleep. If you know you wake up a lot, go to bed a little earlier than the chart says.
How 8 hours fits with sleep cycles
Eight hours is about 5.3 sleep cycles, so the alarm tends to go off early in a new cycle. A cycle averages roughly 90 minutes, and 8 hours divided by 90 minutes is 5.33. Your alarm lands about 30 minutes into cycle 6.
Some people wake from that without any trouble. Others feel groggier than they expect for a full night. If that's you, you have two cycle-aligned options:
| Wake-up time | 7.5 h (5 cycles) | 9 h (6 cycles) |
|---|---|---|
| 6:00 AM | 10:15 PM | 8:45 PM |
| 6:30 AM | 10:45 PM | 9:15 PM |
| 7:00 AM | 11:15 PM | 9:45 PM |
| 7:30 AM | 11:45 PM | 10:15 PM |
Real cycles vary. In one large lab analysis of healthy sleepers, the median cycle ran 96 minutes, and cycle length differed a lot between people. So treat cycle-aligned times as a starting point. If 8 hours feels fine, keep it; more total sleep matters more than hitting a cycle boundary.
For a chart built entirely around cycles, see what time should I go to bed. This page is for people who want a specific number of hours.
Sleep calculator for 9 hours or 7 hours
For 9 hours of sleep, go to bed 9 hours 15 minutes before your alarm; for 7 hours, 7 hours 15 minutes before.
- 9 hours suits people recovering from a hard week, athletes in heavy training, and anyone who finds 8 isn't quite enough. Nine hours is also exactly 6 cycles.
- 7 hours is the floor for adults. It's fine for some people, but if you need an alarm every day and feel sleepy by mid-afternoon, it's probably short for you.
Whichever number you pick, the time that matters most is your wake-up time. Keep it the same every day, including weekends, and let bedtime follow. If your current bedtime is far from the chart, move it 15 minutes earlier every few nights rather than all at once, and use the sleep calculator to check any time that isn't listed.