Sleep Calculator

Overview

Sleep happens in cycles of roughly 90 minutes, each containing four stages: light sleep (N1, N2), deep sleep (N3), and REM. Waking up at the end of a cycle leaves you refreshed; waking mid-cycle (especially mid-deep-sleep) produces grogginess called sleep inertia that can last 30+ minutes. By aligning your alarm to a cycle boundary, you can feel more rested on less total sleep. Most adults need 5–6 complete cycles (7.5 to 9 hours), but waking at a cycle boundary even after a shorter sleep often feels better than waking at random.

How It Works

This calculator works both directions: tell it when you need to wake up, and it suggests bedtimes spaced 90 minutes apart (factoring in average 14 minutes to fall asleep). Or tell it when you're going to bed, and it shows the wake times that align with cycle completions. The math is simple — wake_time − sleep_latency − (cycles × 90 minutes) = bedtime — but applying it consistently changes how rested you feel.

When to Use This

Use this on the night before a long workday or important morning event when sleep timing matters most. Use it when adapting to a new time zone — gradually shifting bedtime by one cycle (90 min) each night is easier than trying to flip the whole schedule. Use it when you're going to bed late and need to choose between waking after 4.5, 6, or 7.5 hours instead of arbitrary alarm times.

Frequently Asked Questions