How this calculator works
Pick an anchor — a wake-up time, a bedtime, or right now — and the calculator counts whole 90-minute cycles forwards or backwards from it, adding the time it takes you to fall asleep. It offers several complete-cycle options so you can pick the one that fits your evening.
The one number it asks you for is how long you take to fall asleep. The 15 minutes average is a reasonable starting point, and it is the default here, but you know your own figure better than any average does — and getting it wrong shifts every suggested time by exactly that much.
How precise is any of this?
Less precise than a calculator implies, and it is worth being straight about that.
A 90-minute cycle is a convenient estimate, not a biological constant. Sleep cycles vary between people and across the same night's sleep. The first cycle is often shorter, while later cycles tend to be longer.
That means the calculator cannot identify your actual sleep-cycle boundaries to the minute. Treat its suggested times as rough targets rather than precise predictions.
Don't treat the suggested times as exact. Sleep cycles vary naturally, so the calculator is best used as a rough planning tool rather than a way to predict the exact moment you will enter or leave a sleep stage.
What this calculator doesn’t model
- Naps. This calculator models one continuous sleep period and does not account for naps or changes in sleep pressure caused by daytime sleep.
- Alcohol. Alcohol, medications, illness, stress, and other factors can affect sleep quality and sleep architecture; the calculator does not model them.
- Pregnancy and other life stages. Sleep patterns and sleep needs can change with age, pregnancy, and other circumstances that aren't represented by a simple cycle model.
- Sleep disorders. Insomnia, sleep apnea, restless legs syndrome, and other conditions can disrupt sleep. A cycle calculator cannot diagnose or account for these conditions.
- Sleep debt. A cycle estimate does not make a short night sufficient.
Frequently asked questions
How long is a sleep cycle?
About 90 minutes on average, which is what this calculator uses. In practice cycles range from roughly 70 to 120 minutes, vary between people and between nights, and lengthen through the night — so the later suggestions carry more uncertainty than the earlier ones.
Is it better to sleep 6 hours or 7.5 hours?
7.5 hours provides more sleep than 6 hours, and getting enough total sleep is more important than matching an exact number of 90-minute cycles. The calculator uses 90-minute cycles as a simple estimate, not as a measure of your actual sleep architecture.
How long should it take me to fall asleep?
Many healthy adults take around 10–20 minutes to fall asleep, although your own normal can vary.
What if I can't hit one of the suggested times?
Use whichever is closest. Given how much cycle length varies, a few minutes either way makes very little difference — and going to bed at a consistent time matters more than hitting any particular minute.
Does this calculator account for naps, alcohol or pregnancy?
No. It models a single continuous night of undisturbed sleep. Alcohol suppresses REM and fragments the second half of the night; pregnancy reduces sleep efficiency and increases waking, especially in the third trimester; naps reduce how much nighttime sleep you need. None of that is captured by cycle arithmetic.
This calculator is for general information and self-tracking. It is not medical advice and cannot diagnose anything. If you regularly struggle to fall asleep, wake unrefreshed, or suspect a sleep disorder, speak to a doctor or a sleep specialist.