Sleep Calculator — Best Time to Sleep or Wake Up
Human sleep runs in cycles of about 90 minutes, and you feel most refreshed when you wake at the end of a cycle rather than in the middle of one. To wake at 7:00 AM, aim to fall asleep at 11:45 PM, 10:15 PM, 8:45 PM or 7:15 PM — allowing about 15 minutes to drift off. Enter your target wake time below to see your options.
What is sleep calculator?
A night of sleep is made up of repeated cycles moving through light sleep, deep sleep and REM sleep. One cycle takes roughly 90 minutes, though it varies between individuals and shifts slightly across the night.
Deep sleep dominates the first half of the night and is when physical restoration happens most actively. REM dominates the second half and is strongly associated with memory consolidation and emotional processing.
Waking during deep sleep is what produces that heavy, disorientated feeling known as sleep inertia. Waking at a cycle boundary avoids most of it, which is the entire premise of this calculator.
How sleep times are calculated
The calculator assumes a 90-minute sleep cycle and adds 15 minutes as the average time it takes to fall asleep. Working backwards from your wake time, it subtracts whole cycles plus that fall-asleep buffer. Waking at the end of a cycle — rather than mid-cycle — reduces sleep inertia and makes you feel considerably more alert.
Bedtime = wake time − (cycles × 90 min) − 15 min
Wake time = bedtime + 15 min + (cycles × 90 min)
Common options: 6 cycles ≈ 9 h · 5 cycles ≈ 7.5 h · 4 cycles ≈ 6 hSource: Based on the 90-minute basic rest-activity cycle (BRAC) model of sleep.
How to use this calculator
- Choose whether you know your wake time or want to sleep now.
- Enter the time.
- Pick the option that gives you the most sleep while still fitting your morning.
- Aim for 5 to 6 cycles (7.5 to 9 hours) as an adult.
- Hold your wake time steady across weekends — a shifting schedule is worse than a slightly short one.
Frequently asked questions
How many sleep cycles do I need?
Most adults feel best with 5 to 6 cycles, which is 7.5 to 9 hours of actual sleep. Consistently running on 4 cycles or fewer is associated with measurable cognitive impairment.
Why do I still feel tired after 8 hours?
You may have woken mid-cycle. It can also reflect sleep quality issues, an inconsistent schedule, alcohol before bed, or an untreated condition such as sleep apnoea. If it persists, speak to a doctor.
Does the 90-minute figure apply to everyone?
It is an average. Individual cycles range from about 70 to 110 minutes and vary across the night. Treat the times as a good starting point and adjust to what actually works for you.
Should I catch up on sleep at weekends?
Some catch-up helps, but a large shift in sleep timing creates social jetlag that makes weekday mornings harder. Keeping wake time within an hour of your weekday time works better for most people.
Does napping break my night sleep?
A short nap of 20 minutes or a full 90-minute cycle is fine. Naps of 40 to 60 minutes wake you mid-cycle and can reduce sleep pressure for the following night.
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- Calorie Calculator — Free calorie calculator using the Mifflin-St Jeor equation. See how many calories you should eat to maintain, lose or gain weight, plus your BMR.
These figures are estimates from population formulas, not medical advice. Individual needs vary. If you are pregnant, managing a medical condition, or under 18, talk to a doctor before changing your diet.