Sleep calculator

How sleep cycles work

Sleep happens in cycles of approximately 90 minutes, moving through light sleep, deep sleep (slow-wave sleep), and REM sleep. Waking mid-cycle — especially during deep sleep — causes sleep inertia: that groggy, disoriented feeling that can persist for 30 minutes or more.

Waking at the end of a complete cycle, when sleep is naturally lightest, feels much more natural. This calculator shows you the ideal wake times that align with cycle boundaries.

The 15-minute fall-asleep buffer

The calculator adds 15 minutes to your bedtime before starting the first cycle. This accounts for the average time it takes a healthy adult to fall asleep (known as sleep latency). If you tend to fall asleep faster or slower, adjust the times by 5–10 minutes accordingly.

How many cycles do you need?

• 4 cycles (6 hours) — minimum for most adults; you'll be functional but not optimal
• 5 cycles (7.5 hours) — ideal for most adults; good cognitive and physical performance
• 6 cycles (9 hours) — recommended after sleep debt, illness, or hard training

The National Sleep Foundation recommends 7–9 hours for adults (18–64). For teenagers, 8–10 hours; for children, more.

Tips for better sleep timing

• Consistent wake times are more important than consistent bedtimes for regulating your circadian rhythm
• Avoid caffeine for 6–8 hours before bed; its half-life is about 5–6 hours
• Exposure to morning sunlight resets your body clock and makes falling asleep easier at night
• Use Left to set a "lights out" countdown for 30 minutes before your target bedtime