Sleep Calculator
Waking mid-deep-sleep is why 8 hours can feel worse than 7.5. This sleep calculator plans your bedtime or wake time on ~90-minute sleep cycles, so the alarm catches you between cycles.
Last updated October 25, 2026
How to use this calculator
- Pick your mode: plan backwards from a fixed wake-up time, or forward from right now.
- Set how long you usually take to fall asleep.
- Choose 6 cycles (9 h) if you can, 5 (7.5 h) as the solid default — 4 is the floor, 3 is a nap.
Why 90 minutes?
A night of sleep runs through repeated cycles of light sleep, deep sleep and REM, each lasting roughly 90 minutes (individual range ~80–120). Waking at a cycle boundary — in light sleep — feels dramatically easier than being torn out of deep sleep mid-cycle, even at the same total duration. That is why 7 hours 30 minutes can beat 8 hours.
| Cycles | Sleep time | Verdict |
|---|---|---|
| 6 × 90 min | 9 h | Plenty — ideal for heavy training blocks and teenagers |
| 5 × 90 min | 7.5 h | The sweet spot for most adults |
| 4 × 90 min | 6 h | Repayable occasionally; not a lifestyle |
| 3 × 90 min | 4.5 h | A structured nap, not a night |
Frequently asked questions
Are sleep cycles really exactly 90 minutes?
No — 80 to 120 minutes is the honest range, and they lengthen with more REM toward morning. Ninety is the useful average for planning wake times.
Is 6 hours of sleep enough?
For most adults, no — 4 cycles is the floor, not a target. Chronic 6-hour nights accumulate impairment that feels normal because you get used to being impaired.
What if I wake up mid-cycle anyway?
Shift your bedtime or alarm by 15-minute steps for a few nights and watch how you feel on waking. The cycle estimate gets you close; your body confirms the landing.
Does this work for naps?
A 20-minute power nap stays in light sleep — set a plain 20-minute alarm. For a full-cycle nap (90 min), you wake between cycles the same way as at night.