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Sleep Calculator
Find the best times to wake up or fall asleep based on 90-minute sleep cycles. Enter your wake-up time or bedtime and we'll show you when to sleep or wake for maximum energy.
I want to wake up at
:
Why 90-minute cycles?
Sleep occurs in repeating cycles of roughly 90 minutes, each moving through light sleep, deep (slow-wave) sleep, and REM sleep. Waking at the end of a cycle — when sleep is lightest — feels dramatically more refreshing than waking mid-cycle from deep sleep.
Times include a 14-minute sleep onset allowance — the average time it takes to fall asleep after getting into bed.
For informational purposes only
Sleep cycle length varies between individuals and across the night. These are estimates based on population averages. If you have sleep difficulties, consult a GP or sleep specialist.
How to use the Sleep Calculator
Choose your mode
Select 'I want to WAKE UP at…' if you know when you need to be up and want to find the best bedtime. Select 'I'm going to BED at…' if you know when you'll sleep and want to find optimal wake times.
Enter your time
Type your hour and minutes, then select AM or PM. Or click 'Set to now' to fill in the current time automatically.
Review the sleep options
Five options appear showing times aligned to complete sleep cycles. Each card shows the time, number of cycles, total sleep hours, and a quality rating from 'Too short' to 'Extended'.
Aim for the Ideal option
The 'Ideal' card (7.5 hours, 5 cycles) is highlighted with a green border and a Recommended badge. Most adults need 7–9 hours, and 5 cycles puts you squarely in that range.
About this Sleep Calculator
Sleep quality matters as much as sleep quantity. Waking mid-cycle from deep sleep leaves you groggy and disoriented — a feeling called sleep inertia. Our sleep calculator helps you schedule your bedtime or wake-up time to align with the natural end of a 90-minute sleep cycle, when sleep is lightest and waking feels easiest. Sleep cycles progress through four stages: NREM Stage 1 (light sleep, easily woken), NREM Stage 2 (body temperature drops, heart rate slows), NREM Stage 3 (deep slow-wave sleep, hardest to wake from), and REM sleep (rapid eye movement, when most dreaming occurs). Each full cycle lasts around 90 minutes, though cycle length varies slightly across the night — early cycles have more deep sleep, later cycles more REM. The 14-minute sleep onset latency included in all calculations reflects the average time it takes a healthy adult to fall asleep after getting into bed. If you fall asleep faster or slower than average, mentally adjust the times. For adults, 7–9 hours (4–6 complete cycles) is the range recommended by the American Academy of Sleep Medicine and the Sleep Research Society.
Frequently Asked Questions
A complete sleep cycle lasts approximately 90 minutes and includes light sleep, deep sleep, and REM sleep. Most adults complete 4–6 cycles per night. Waking up at the end of a cycle feels more natural and refreshing.
Adults generally need 7–9 hours of sleep per night. Teenagers need 8–10 hours, and younger children need more. Consistently sleeping fewer than 7 hours is associated with a range of health issues.
Waking during deep sleep (typically in the first half of the night) causes sleep inertia — that groggy, disoriented feeling. Our sleep calculator helps you schedule wake-up times that align with the end of a cycle to minimise this effect.
REM (Rapid Eye Movement) sleep is the stage where most vivid dreaming occurs. The brain is highly active — almost as active as when awake — while the body's muscles are temporarily paralysed. REM sleep is important for memory consolidation, emotional processing, and creativity. It occurs in greater proportions later in the night, which is why cutting sleep short disproportionately reduces REM.
Research suggests that 'sleep banking' at weekends can partially repay short-term sleep debt and improve performance, but it doesn't fully reverse the metabolic and cognitive effects of chronic sleep deprivation. Consistent sleep timing — the same bedtime and wake time every day — is more beneficial than irregular catch-up sleep.