Sleep Cycle Calculator
Enter when you turned the lights out and when you woke, plus how long you took to fall asleep. You get the number of 90-minute cycles you completed, an estimated split into light, deep and REM sleep, and whether the alarm caught you mid-cycle.
Bathroom trips, tossing, the 3 a.m. stare.
Time asleep
7 h 5 min
7 h 30 min in bed · 94% efficiency
Cycles completed
4
plus 1 h 5 min of a 90-min cycle
Vs. your need
7–9 h
within range
Woke mid-cycle?
Likely
deep-sleep window of the last cycle
Estimated stage breakdown
| Stage | Share | Time | What it does |
|---|---|---|---|
| N1 · Light (drowsy) | 5% | 21 min | The doorway; easily woken. |
| N2 · Light | 48% | 3 h 24 min | Memory consolidation, sleep spindles; most of the night. |
| N3 · Deep (slow-wave) | 22% | 1 h 34 min | Physical restoration, growth hormone; front-loaded in the first cycles. |
| REM | 25% | 1 h 46 min | Dreaming, emotional processing; back-loaded, longest before waking. |
Stage shares are adult averages from sleep-lab studies; a wearable estimates them from movement and heart rate and is usually within a few percent on light vs. deep, less reliable on REM.
Cycles and stages by time asleep
Typical adult stage shares applied to each total: 5% N1, 48% N2, 22% N3 (deep), 25% REM. Real nights vary; the shape is what to read.
| Asleep | Full cycles | Light (N1+N2) | Deep (N3) | REM |
|---|---|---|---|---|
| 5 h | 3 + 30 min | 2 h 39 min | 1 h 6 min | 1 h 15 min |
| 6 h | 4 | 3 h 11 min | 1 h 19 min | 1 h 30 min |
| 6 h 30 min | 4 + 30 min | 3 h 27 min | 1 h 26 min | 1 h 38 min |
| 7 h | 4 + 60 min | 3 h 43 min | 1 h 32 min | 1 h 45 min |
| 7 h 30 min | 5 | 3 h 59 min | 1 h 39 min | 1 h 53 min |
| 8 h | 5 + 30 min | 4 h 14 min | 1 h 46 min | 2 h |
| 8 h 30 min | 5 + 60 min | 4 h 30 min | 1 h 52 min | 2 h 8 min |
| 9 h | 6 | 4 h 46 min | 1 h 59 min | 2 h 15 min |
Why the last cycle matters most
Stage shares are averages over the whole night, but they are not spread evenly. Deep sleep peaks in cycle one and is nearly gone by cycle four; REM does the opposite. On a 7 h 30 min night the final cycle might be 10 minutes of deep sleep and 50 of REM. Lose that cycle to an early alarm and you have lost perhaps a third of the night's REM while keeping almost all its deep sleep. That is the physiology behind "I slept six hours and my memory is shot but my legs feel fine."
Sleep efficiency
Time asleep divided by time in bed. 85% and above is the clinical benchmark for good sleep; below 80% for weeks is a marker of insomnia. The calculator subtracts your time to fall asleep and time awake during the night, which is what a sleep clinic's diary does. If your efficiency is low, the counter-intuitive fix is to spend less time in bed, so the time you do spend is more consolidated -- the core of CBT-I.
Frequently Asked Questions
How many sleep cycles should I get a night?
Four to six for an adult: five 90-minute cycles is 7.5 hours, six is 9. Four (6 hours) is the floor most people can sustain without a growing debt, and three is a bad night whatever the alarm says.
How much REM sleep should I get?
About 20 to 25% of the night in adults, so 90 to 110 minutes on a 7.5-hour night. REM is back-loaded: the first REM period lasts a few minutes, the last one can run 30 to 60. That is why cutting the last hour of sleep removes a disproportionate share of REM, and why wearables show low REM after short nights.
How much deep sleep is normal?
Roughly 15 to 25% of the night, 60 to 110 minutes, and it declines with age -- a 20-year-old might get 25%, a 60-year-old 10 to 15%. Deep sleep is front-loaded into the first two cycles, so it is the most protected stage: go to bed late and you still get most of it, wake early and you lose REM instead.
Why do I feel worse after 8 hours than after 6?
Usually because the 8-hour alarm landed in deep sleep, mid-cycle, and the 6-hour one landed at a cycle boundary in light sleep. Sleep inertia from a deep-sleep wake can last 30 to 60 minutes. The fix is not less sleep; it is an alarm that matches a boundary, which the bedtime calculator works out.
Are the stage numbers from my watch accurate?
Roughly, for light versus deep. Consumer wearables estimate stages from movement and heart-rate variability and agree with lab polysomnography about 60 to 70% of the time on any given epoch; they are decent on total sleep, fair on deep, and weakest on REM. Use the trend over weeks, not last night's number.
These calculators use published sleep-science guidelines and are for general guidance. They are estimates, not medical advice: persistent insomnia, loud snoring or gasping in sleep, or daytime sleepiness that affects driving are reasons to see a doctor, not to adjust a bedtime.