Sleep calculator

What this calculator does

It works backwards from your wake-up time and suggests bedtimes that fit whole 90-minute sleep cycles, plus the time you usually need to fall asleep.

The idea is simple: waking in the middle of a deep-sleep cycle can feel worse than waking a little earlier at the end of one. Aligning the alarm with cycle boundaries is a planning trick, not a guarantee.

The arithmetic

Bedtime = wake time − (cycles × 90 minutes) − time to fall asleep

For a 07:00 alarm and 15 minutes to fall asleep:

• 6 cycles: 21:45
• 5 cycles: 23:15
• 4 cycles: 00:45

Each option is 90 minutes apart. The result wraps around midnight when needed.

A worked example

Wake at 06:30, take 20 minutes to fall asleep:

• 6 cycles: 06:30 − 9 h − 20 min = 21:10
• 5 cycles: 22:40
• 4 cycles: 00:10

Aim for the 5- or 6-cycle option; four cycles is six hours, below the seven to nine hours most adults need.

What the cycles actually are

Sleep moves through stages, and a full cycle is roughly 90 minutes on average. Deep sleep is concentrated early in the night, REM later. That is why cutting sleep short affects people differently from shifting it. The 90-minute figure is a useful average, not a fixed rule.

What this calculator does not do

It does not diagnose insomnia, sleep apnoea or any sleep disorder, and it does not measure your actual cycles. It does not replace a sleep study or clinical advice. If sleep problems persist, talk to a clinician.

Light and consistency

The arithmetic assumes you fall asleep at the time you get into bed. Light, screens, caffeine and a variable schedule change that. A consistent wake time is usually more useful than a perfect bedtime.