Why the calculator uses a range
Sleep moves through non-REM and REM phases repeatedly. A cycle often restarts after roughly 80 to 100 minutes, and its composition changes through the night. That is why a single fixed 90-minute block creates false precision.
Time in bed is not time asleep
A bedtime calculation also cannot know how long you will take to fall asleep, how often you will wake, or whether pain, alcohol, caffeine, shift work, age or a sleep disorder is changing sleep structure. Waking near a predicted boundary does not guarantee that you will feel refreshed.
Use it as a planning aid
Choose a realistic window that protects enough total sleep and a consistent routine. Persistent insomnia, loud snoring with breathing pauses, severe daytime sleepiness or safety problems need professional assessment. A cycle calculator cannot diagnose or rule out a sleep disorder.