Bedtime Calculator
Working Backward From a Fixed Wake-Up Time
Most people don't choose their wake-up time freely — work, school, or a flight sets it, and bedtime is the only variable left to control. This calculator starts from that fixed wake-up time and counts backward in full 90-minute sleep cycles, surfacing several bedtime options rather than a single number, so you can weigh a fuller night against getting to bed sooner.
The Formula
The 15-minute subtraction accounts for the time it typically takes to fall asleep after getting into bed, so the cycle count reflects actual sleep time rather than time spent lying awake.
Where This Matters
- Fixed morning commitments — an early flight, exam, or shift start makes wake-up time non-negotiable; this reframes the decision around bedtime instead.
- Avoiding a mid-cycle alarm — a wake-up that falls in the middle of a deep-sleep phase tends to produce more sleep inertia than one timed to a cycle boundary.
- Weighing trade-offs — seeing 4, 5, and 6 cycle options side by side makes it easier to judge whether staying up later is really worth the lost cycle.
Example: Waking at 6:30 AM
| Cycles | Total sleep | Bedtime |
|---|---|---|
| 6 | 9 hrs | 09:15 PM |
| 5 | 7.5 hrs | 10:45 PM |
| 4 | 6 hrs | 12:15 AM |
| 3 | 4.5 hrs | 01:45 AM |
90 minutes is an average cycle length used for planning; individual cycles run somewhat shorter or longer, particularly earlier versus later in the night.
How to Use This Calculator
- Enter your target wake-up time in the Wake-Up Time field.
- Select Calculate to see bedtime options for 3 through 6 complete sleep cycles.
Related Calculations
Already know your bedtime instead? Use the Wake Up Time Calculator, or compare the result against age-based guidance with the Sleep Need Calculator.