Sleep arrives in cycles, not a flat block
A night's sleep moves through repeating cycles of light sleep, deep sleep and REM. Each cycle runs roughly 90 minutes, though 70 to 120 minutes is normal, and the composition shifts across the night — deep sleep dominates early cycles, REM dominates the later ones.
Waking at the end of a cycle, in light sleep, generally feels easier than being pulled out of deep sleep mid-cycle. That is the whole idea behind counting backwards in 90-minute blocks.
The 15 minutes is sleep latency —
the time it takes to actually fall asleep
Bedtimes by wake time
Six cycles is nine hours of sleep; five is seven and a half; four is six.
| Wake at | 6 cycles (9h) | 5 cycles (7.5h) | 4 cycles (6h) |
|---|---|---|---|
| 5:30 am | 8:15 pm | 9:45 pm | 11:15 pm |
| 6:00 am | 8:45 pm | 10:15 pm | 11:45 pm |
| 6:30 am | 9:15 pm | 10:45 pm | 12:15 am |
| 7:00 am | 9:45 pm | 11:15 pm | 12:45 am |
| 7:30 am | 10:15 pm | 11:45 pm | 1:15 am |
| 8:00 am | 10:45 pm | 12:15 am | 1:45 am |
Five cycles suits most adults. Four is a short night to be used occasionally rather than planned around.
How much sleep by age
| Age group | Recommended nightly |
|---|---|
| Newborn (0–3 months) | 14–17 hours |
| Infant (4–11 months) | 12–15 hours |
| Toddler (1–2 years) | 11–14 hours |
| Preschool (3–5) | 10–13 hours |
| School age (6–13) | 9–11 hours |
| Teen (14–17) | 8–10 hours |
| Adult (18–64) | 7–9 hours |
| Older adult (65+) | 7–8 hours |
Individual need varies within these ranges. The practical test is daytime function: consistently needing an alarm to wake, feeling drowsy mid-afternoon, or sleeping several hours longer at weekends all suggest a shortfall on weeknights.
Why the 90-minute figure is approximate
Cycle length is not fixed. It differs between people, changes across the night, and shifts with sleep debt, alcohol and stress. Treating 90 minutes as exact will not reliably land your alarm in light sleep.
What matters more than cycle arithmetic:
- Consistency. The same wake time daily, weekends included, does more for how you feel than optimising any single night.
- Total duration. Seven and a half hours at a slightly awkward point in a cycle beats six hours perfectly timed.
- Light exposure. Bright light shortly after waking and dim light in the evening anchor the body clock more powerfully than bedtime maths.
Sleep latency
The 15-minute allowance in the formula is a healthy average. Falling asleep in under five minutes routinely tends to indicate significant sleep debt. Regularly taking more than 30 minutes points the other way — often too little time awake before bed, late caffeine, or an inconsistent schedule.
Caffeine has a half-life of roughly five to six hours, so a 3pm coffee can still have a quarter of its dose active at 9pm. Alcohol shortens time to sleep but suppresses REM and fragments the second half of the night.
If sleep problems persist for weeks despite a consistent schedule, that is worth raising with a doctor rather than solving with timing tweaks — insomnia and sleep apnoea both need proper assessment.
Work out your own bedtimes in the sleep calculator, or check resting heart rate zones if you are tracking recovery alongside sleep.
Run your own numbers
FAQ
How long is a sleep cycle?
Is 6 hours of sleep enough?
Why do I wake up tired after 8 hours?
Does the 90-minute rule actually work?
How late can I drink coffee?
Sources
Primary references used for the figures and rules on this page.
- About Sleep — CDC
- Sleep Deprivation and Deficiency — NIH — NHLBI