How the sleep calculator works
I'm Sleepy works forwards from a bedtime or backwards from a wake-up time. It is deliberately a planning tool, not a sleep tracker or a measurement of your actual sleep stages.
The calculation
By default, the calculator uses 90-minute cycles and allows 15 minutes to fall asleep. Both values can be changed in Settings. It then shows one through six complete-cycle estimates, with the longer options presented first.
Why 90 minutes is only an estimate
Sleep alternates between non-REM and REM phases. The US National Heart, Lung, and Blood Institute says a cycle usually restarts every 80 to 100 minutes, with four to six cycles in a typical night. That range is why the calculator lets you adjust the cycle length instead of pretending every person and every night follows an exact timer.
Source: NHLBI — phases and stages of sleepWhat the results cannot tell you
A clock calculation cannot know when you actually fall asleep, which stage you are in, whether you wake during the night, or how your circadian rhythm affects you. Treat the times as convenient options, not predictions or medical advice.
Completing a neat number of estimated cycles is not a substitute for getting enough sleep. The CDC recommends at least seven hours per day for adults. If sleep problems are persistent or affect daily life, speak to a qualified healthcare professional.
Source: CDC — sleep in adultsRecommend the calculator
Writing a wellbeing, education or productivity resource? You are welcome to link directly to the free calculator. This short snippet keeps the recommendation clear:
<a href="https://www.sleepy.im/">Free sleep calculator by I'm Sleepy</a>