The Architecture of a Night
Each cycle descends from light sleep through deep slow-wave sleep and back up into REM, then repeats. Deep sleep dominates the first half of the night, that's where physical repair concentrates, while REM bunches toward morning.
Being wrenched out of deep sleep causes sleep inertia: the foggy, heavy grogginess that can last an hour. Waking at a boundary, where sleep is light, avoids most of it.
Timing the Alarm
Count in 90-minute blocks from lights-out, plus 10–20 minutes of sleep latency. Bed at 22:30 means natural wake points around 06:15 (5 cycles) and 07:45 (6), and a miserable 06:45 alarm.
Individual cycles range 80–120 minutes, so treat the math as a starting point. If a fixed alarm always feels brutal, shift bedtime by 30 minutes and test.
Hygiene That Actually Works
The strongest single habit is a fixed wake time, it anchors the circadian rhythm and makes bedtime self-regulating. Add bright morning light, dim evenings, and a cool dark room.
Caffeine after early afternoon, alcohol (which fragments REM), and screens in bed each cost measurable sleep quality. Consistency beats all gadgetry.
Sources
FAQ
What is the best time to wake up?
At the end of a 90-minute cycle, for a 22:30 lights-out, that's about 06:15 or 07:45, not 07:00.
Is the 90-minute rule real?
Cycles average ~90 minutes but vary 80–120 minutes per person. The rule is a planning heuristic, validate it against how you actually feel.
Why do I feel groggy even after 8 hours?
Waking during deep sleep causes sleep inertia. Shift the alarm ±30 minutes and compare how you feel at the same total duration.
Please read this first: these results are educational estimates. For real decisions, talk to a qualified professional.