For most adults, 8 hours of sleep is slightly better than 7, but both are inside the healthy range. Sleep experts recommend 7 to 9 hours for adults, so 7 is the floor, not the target. If you're choosing a bedtime, 7.5 hours (5 full sleep cycles) is often the best of both: close to 8 hours of sleep, and timed so you wake at the end of a cycle.
Free toolSleep CalculatorBest bedtime or wake-up time, based on 90-minute sleep cycles.What the guidelines say
The American Academy of Sleep Medicine and Sleep Research Society recommend that adults sleep 7 or more hours a night on a regular basis. The same panel said more than 9 hours may be appropriate for young adults, people recovering from sleep debt and people who are ill, which is why the healthy range is usually given as 7 to 9 hours.
That means 7 hours counts as enough for many people, but it leaves no room for error. A night that runs 30 minutes short, or 20 minutes of lying awake, drops you below the line.
What the research shows
A 2003 study at Walter Reed Army Institute of Research put volunteers on 3, 5, 7 or 9 hours in bed for a week. Only the 9-hour group stayed at their normal reaction speed. The 7-hour group slowed down, settled at a lower level, and hadn't fully bounced back after three nights of normal sleep.
Keep in mind that was time in bed, not sleep. Seven hours in bed usually means well under 7 hours of actual sleep, which is why 7 hours in bed is not the same as 7 hours asleep.
What about more sleep being bad? Large population studies have linked both very short and very long sleep with higher death rates. But those studies rely on people reporting their own sleep, and long sleep may be a sign of an existing illness rather than its cause. It's not a reason to cut back from 8 hours if 8 is what your body wants.
The sleep-cycle math
Sleep runs in cycles of about 90 minutes. Waking at the end of a cycle, when sleep is lighter, usually feels easier than being pulled out of deep sleep.
| Sleep | Cycles | Where you wake |
|---|---|---|
| 7 hours | 4.7 | Partway through cycle 5 |
| 7.5 hours | 5 | End of a cycle |
| 8 hours | 5.3 | 30 minutes into cycle 6 |
| 9 hours | 6 | End of a cycle |
This is why some people feel groggier after 8 hours than after 7.5. They got more sleep, but the alarm caught them early in a new cycle. Real cycles vary from about 70 to 120 minutes, so this isn't exact for everyone, but it's a useful starting point.
Bedtimes for 7, 7.5, 8 and 9 hours
Each bedtime includes 15 minutes to fall asleep.
| Wake-up time | 7 hours | 7.5 hours | 8 hours | 9 hours |
|---|---|---|---|---|
| 6:00 AM | 10:45 PM | 10:15 PM | 9:45 PM | 8:45 PM |
| 6:30 AM | 11:15 PM | 10:45 PM | 10:15 PM | 9:15 PM |
| 7:00 AM | 11:45 PM | 11:15 PM | 10:45 PM | 9:45 PM |
| 7:30 AM | 12:15 AM | 11:45 PM | 11:15 PM | 10:15 PM |
For any other wake-up time, the sleep calculator gives you cycle-timed bedtimes in one tap.
So which should you aim for?
- Aim for 7.5 hours as your everyday target. It's comfortably inside the range and times your wake-up to a cycle boundary.
- Go for 9 hours (6 cycles) if you're training hard, recovering from illness, or paying back a rough week.
- Treat 7 hours as your minimum, not your goal. If you regularly get 7 or less and need caffeine to get through the morning, you probably need more.
- Skip the in-between times if you often wake groggy. Eight hours is great if you wake easily. If not, try 7.5 or 9 instead.
What you gain in the extra hour
The last cycles of the night are heavy on REM sleep, which is tied to memory, learning and mood. Going from 7 to 8 hours mostly adds late-night REM. Learn more in what is REM sleep.
If you've been living on 7 or less, check how much you've fallen behind with the sleep debt calculator. And if 7 hours already feels like a stretch, here's why 6 hours isn't enough either.