How to use this calculator
Compare overnight clock-time duration or illustrative bedtimes and wake times based on fixed 90-minute intervals. Choose the mode and enter the requested time.
How the estimate is calculated
The duration mode treats a wake time earlier than bedtime as occurring the next day. Cycle-based options add or subtract fixed 90-minute intervals using the tool’s stated assumptions. They do not observe when you actually fall asleep or enter a sleep stage.
What the estimate cannot tell you
Real sleep cycles vary in length across the night and between people. A clock-time calculator cannot identify REM stages, diagnose a sleep disorder or guarantee that waking at a displayed time will feel easier. Do not shorten sleep merely to fit a cycle count. Schedule and quality inputs do not turn these estimates into a physiological measurement.
Worked example
From 23:00 to 07:00 is 8 hours of clock time. Five fixed 90-minute intervals instead total 7 hours 30 minutes; that numerical difference is not evidence about your actual sleep stages.