Sleep Calculator
Find the best time to wake up or go to bed based on 90-minute sleep cycles. Wake up feeling refreshed, not groggy.
Sleep Cycle Calculator
Go to bed at...
3:46 am
Very short — emergency only
2 cycles (3 hrs)
2:16 am
Short — not recommended
3 cycles (4.5 hrs)
12:46 am
Minimum — may feel groggy
4 cycles (6 hrs)
11:16 pm
Good — most adults feel rested
5 cycles (7.5 hrs)
Recommended9:46 pm
Ideal — full rest
6 cycles (9 hrs)
RecommendedThe 4 Stages of Sleep
| Stage | Type | Duration | Function |
|---|---|---|---|
| Stage 1 | Light NREM | 1–7 min | Transition to sleep |
| Stage 2 | Light NREM | 10–25 min | Body temperature drops, heart rate slows |
| Stage 3 | Deep NREM | 20–40 min | Physical restoration, immune function |
| Stage 4 | REM | 10–60 min | Memory consolidation, emotional processing |
Sleep Tips
Adults need 7–9 hours per night (NHS)
Avoid screens 1 hour before bed
Keep your bedroom cool (16–18°C is ideal)
Avoid caffeine after 2pm
A consistent sleep schedule is more important than total hours
Why Sleep Cycles Matter
Sleep is not a single continuous state — it is a series of repeating cycles, each lasting approximately 90 minutes. Each cycle moves through four stages: two stages of light sleep, one stage of deep (slow-wave) sleep, and a final stage of REM (rapid eye movement) sleep. Waking up during deep sleep or REM sleep causes the groggy, disoriented feeling known as sleep inertia. Waking up at the end of a complete cycle, when sleep is naturally lightest, leaves you feeling alert and refreshed.
The NHS recommends that adults get between 7 and 9 hours of sleep per night. This corresponds to 5 or 6 complete 90-minute cycles. Most people need between 5 and 6 cycles to feel fully rested, though individual needs vary. This calculator accounts for the average 14 minutes it takes most adults to fall asleep, so the bedtimes shown are the times you should actually get into bed, not the time you need to be asleep.
If you cannot achieve a full 5 or 6 cycles, it is better to aim for a complete number of cycles (for example, 4 cycles = 6 hours) than to sleep for a non-cycle-aligned duration (such as 6.5 hours), as the latter is more likely to leave you waking during deep sleep.