Wake Up Time Calculator

How Sleep Cycles Work

Sleep occurs in cycles of approximately 90 minutes, each containing light sleep (Stage 1–2), deep sleep (Stage 3–4, "slow wave"), and REM sleep. Waking up at the end of a cycle — when you are in light sleep — leaves you feeling alert. Waking during deep sleep produces "sleep inertia" — that groggy, foggy feeling that can persist for 30+ minutes.

Cycles vs. Hours

• 4 cycles — 6 hours sleep (minimal, OK for one night)
• 5 cycles — 7.5 hours (good for most adults)
• 6 cycles — 9 hours (great for athletes, recovery, teens)

Tips for Better Sleep Cycles

• Keep a consistent sleep/wake schedule, even on weekends
• Avoid caffeine after 2 PM (half-life ≈ 6 hours)
• Dim screens and lights 1 hour before bed (low blue light)
• Keep bedroom 60–67 °F (16–19 °C)
• Get 10–30 min of morning sunlight to anchor your circadian rhythm