Sleep Cycle Calculator
Calculate optimal wake times based on 90-minute sleep cycles to minimize sleep inertia.
The Physiology of Sleep Inertia
Waking up in the middle of Stage 3 (Deep Sleep) or REM sleep causes "sleep inertia"—that groggy, disoriented feeling that can last for hours.
A normal sleep cycle lasts approximately 90 minutes. The brain transitions from light sleep to deep sleep, into REM, and back to light sleep. By timing your alarm to coincide with the end of a 90-minute cycle (during light sleep), you dramatically reduce the friction of waking up.
Optimal Durations
Most healthy adults require 5 or 6 complete cycles per night:
- 5 cycles = 7.5 hours
- 6 cycles = 9.0 hours