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Sleep is not one uniform state. Through the night the brain repeats a loop — light sleep, then deep slow-wave sleep, then REM — and that full cycle averages about 90 minutes. A night normally contains four to six of them. Deep sleep is concentrated in the first half of the night and REM in the second, so the last hour is not interchangeable with the first.
The practical consequence is that when you wake matters, not just how long you slept. An alarm that fires in the middle of deep sleep drags you up through the lighter stages, and the resulting grogginess — sleep inertia — can take half an hour to clear. Waking in the last few minutes of a cycle, when sleep has already lightened, usually feels noticeably easier. Every calculator on this site counts back in 90-minute blocks from a wake-up time and adds 15 minutes for the time it takes to fall asleep.
| Age group | Recommended sleep |
|---|---|
| Adult (18-60 years) | 7 or more hours |
| Older adult (65+) | 7-8 hours |
| Teen (13-17 years) | 8-10 hours |
| School age (6-12 years) | 9-12 hours |
| Preschool (3-5 years) | 10-13 hours |
| Toddler (1-2 years) | 11-14 hours |
| Infant (4-11 months) | 12-16 hours |
| Newborn (0-3 months) | 14-17 hours |
| What you want to know | Use |
|---|---|
| What time to go to bed, given my alarm | Sleep Calculator |
| How many cycles I will complete tonight | Sleep Cycle Calculator |
| Bedtime for a set number of hours | Bedtime Calculator |
| How much sleep my age group needs | How Many Hours of Sleep Do I Need? |
| A specific wake-up time, pre-calculated | The pages below, one per wake time |
| Complete cycles | Time asleep | Time in bed (with 15 min to fall asleep) |
|---|---|---|
| 4 | 6 h 00 m | 6 h 15 m |
| 5 | 7 h 30 m | 7 h 45 m |
| 6 | 9 h 00 m | 9 h 15 m |
Five cycles is the usual target for an adult: 7.5 hours asleep, which lands inside the 7–9 hour guidance. A night is normally 4–6 cycles, and the cycles are not identical — deep sleep is concentrated in the first half of the night and REM sleep in the second half, which is why cutting the last hour or two costs a disproportionate share of REM.
You do not fall asleep the moment you get into bed. Sleep latency for a healthy adult is typically 10–20 minutes, so every bedtime this calculator returns has 15 minutes built in. Planning to be asleep at 11:00 PM therefore means getting into bed at about 10:45 PM. Falling asleep in under five minutes is not a sign of good sleep — it usually means you are sleep-deprived.
Time in bed also is not the same as time asleep. Brief awakenings through the night (1–3 is normal, a few minutes each) mean a full night in bed usually yields 30–60 minutes less actual sleep, so if you need 7.5 hours of sleep, plan roughly 8 hours in bed.