Find the perfect bedtime or wake-up time based on 90-minute sleep cycles.
The Sleep Calculator helps you find the perfect bedtime or wake-up time by working backwards or forwards from your target time using 90-minute sleep cycles. The human sleep cycle lasts about 90 minutes — waking up at the end of a cycle leaves you feeling refreshed and alert. Waking mid-cycle causes grogginess (sleep inertia). This tool calculates the optimal times so you always wake at the right moment.
Find the perfect bedtime when you have an early meeting, flight, or commute — ensuring you wake refreshed rather than groggy.
Calculate optimal sleep windows when working irregular hours to maximise sleep quality within limited time.
Figure out the latest you can sleep while still waking at a specific time without feeling exhausted.
Plan bedtimes for children or adjust the family schedule around school drop-off and activities.
A full sleep cycle consists of four stages: light sleep (N1), deeper sleep (N2), deep slow-wave sleep (N3), and REM (rapid eye movement) sleep. Together these stages take approximately 90 minutes. Completing full cycles maximises the restorative benefits of sleep.
On average, it takes a person 15 minutes to fall asleep after getting into bed. The calculator accounts for this so the recommended times are based on when you actually fall asleep, not just when you lie down.
Adults need 5–6 full sleep cycles per night, equating to 7.5–9 hours of sleep. Consistently getting fewer than 4 cycles (6 hours) is associated with reduced cognitive function and long-term health risks.
Sleep cycle length varies between individuals (typically 80–110 minutes) and changes throughout the night. The 90-minute figure is the widely accepted average. Use these times as a guide rather than a rigid rule.
Yes, completely free with no account required. Calculate optimal sleep times as often as you need.