The calculation is intentionally simple, visible and adjustable. It does not attempt to measure your sleep stages or predict exact physiology.
The default cycle length is 90 minutes. NHLBI explains that sleep cycles restart approximately every 80–100 minutes and that people usually experience four to six cycles per night.
Because an individual's cycles are not fixed, the flagship calculator allows the estimate to be changed from 80 to 100 minutes.
The default is 15 minutes, but it is a planning convention rather than a measurement of you. You can set the buffer anywhere from zero to 60 minutes.
Age labels use published CDC duration guidance. The age control changes the contextual labels shown beside results; it does not claim to predict a person's individual sleep need.
Actual sleep architecture changes across the night and varies by person. A clock calculator cannot detect REM, deep sleep, awakenings, sleep disorders or sleep quality.