Sleep Calculator
Find optimal bedtime based on 90-minute sleep cycles.
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How This Sleep Calculator Works
This calculator uses established scientific formulas validated in peer-reviewed literature. All calculations run locally in your browser — no data is sent to any server.
Sleep Recommendations by Age
| Age Group | Recommended Hours | Cycles |
|---|---|---|
| Teens (14–17) | 8–10 hours | 5–7 cycles |
| Adults (18–64) | 7–9 hours | 5–6 cycles |
| Older Adults (65+) | 7–8 hours | 5 cycles |
Poor sleep elevates cortisol, increasing BMI and body fat. It also reduces BMR and impairs exercise recovery. If tracking fitness, sleep quality is as important as nutrition.
The Science of Sleep Cycles
Each 90-minute sleep cycle progresses through four stages. Stage 1 (N1) is the transition from wakefulness — light sleep lasting 1-7 minutes. Stage 2 (N2) is where sleep spindles and K-complexes occur; this is where you spend approximately 50% of total sleep time. Stage 3 (N3) is deep or "slow-wave" sleep — the most restorative phase for physical recovery, immune function, and growth hormone release. REM sleep is when most dreaming occurs and is critical for memory consolidation, emotional regulation, and learning.
The composition of these stages changes across the night. Early cycles are dominated by deep sleep (N3), while later cycles have longer REM periods. This is why waking up after 6 hours (4 complete cycles) often feels better than waking up after 7 hours (interrupting the 5th cycle during deep sleep), even though you've slept less.
Sleep and Body Composition
Sleep deprivation has profound effects on body composition and metabolism. Research from the University of Chicago found that participants on a calorie-restricted diet who slept 5.5 hours per night lost 55% more lean muscle mass and 60% less fat compared to those sleeping 8.5 hours — despite eating the same number of calories. Sleep restriction also increases ghrelin (the hunger hormone) by 28% and decreases leptin (the satiety hormone) by 18%, leading to an average increase of 385 calories consumed per day.
If you're tracking body fat, lean mass, or BMI, sleep quality is as important as your calorie intake and macros. Poor sleep also elevates cortisol, promotes visceral fat storage (check with waist-to-hip ratio), reduces metabolic rate, and impairs exercise performance — reducing both calories burned and strength output.
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