90-Minute REM Cycle Science
Sleep Cycle & Bedtime Calculator
Waking up in the middle of a deep sleep cycle causes morning grogginess (sleep inertia). Calculate optimal bedtimes to wake up feeling refreshed and energized.
What time do you need to wake up?
The 90-Minute Rule:
A complete human sleep cycle lasts approximately 90 minutes, moving through Light Sleep (NREM 1 & 2), Deep Slow-Wave Sleep (NREM 3), and Dream Sleep (REM). If your alarm rings during Deep Sleep, your brain feels disoriented for hours. Waking up at the completion of a cycle guarantees instant alertness.