Why timing beats total hours
You've probably slept eight full hours and still woken up feeling wrecked. The culprit is usually timing, not duration. Sleep moves through cycles of about 90 minutes — light sleep, deep sleep, then REM — and being pulled out of deep sleep by an alarm leaves you with sleep inertia, that thick, groggy fog that can linger for an hour.
Waking at the end of a cycle sidesteps that. Instead of setting your alarm to a round number of hours, this calculator counts in whole 90-minute blocks and adds roughly 15 minutes to drift off, so you rise in the light phase between cycles. Five or six cycles a night suits most adults. It's a small shift in when you set the alarm, and it can change how the whole morning feels.
