Sleep doesn't happen in one block — it happens in roughly 90-minute cycles through light sleep, deep sleep and REM. Waking at the end of a cycle, when you're already in light sleep, feels far easier than being yanked out of deep sleep at an arbitrary point. This calculator picks bedtimes (or wake times) so the alarm lands at a cycle boundary instead of in the middle of one.
It also adds 14 minutes for average sleep latency — the time it actually takes to fall asleep after lying down. So a "go to bed at 22:46" suggestion really means "be in bed by 22:46 to fall asleep around 23:00". Most adults function best on 5 or 6 cycles; the calculator highlights both.
A typical adult cycle through light sleep, deep sleep and REM lasts roughly 90 minutes, repeating four to six times a night. Waking at the end of a cycle (light sleep) feels far more natural than waking in the middle of deep sleep, even with the same total hours.
Sleep onset latency — the average time it takes a healthy adult to fall asleep after lying down. Adding it shifts the calculation from "when I close my eyes" to "when sleep actually starts", which is what your sleep cycles run from.
Often, yes — 7.5 hours is exactly five 90-minute cycles. 8 hours puts your alarm in the middle of cycle 6, which is why "8 hours" can feel groggier than "7.5 hours" for the same night.
It's an average. Real cycles range 70–120 minutes and lengthen toward morning. The calculator gives a best-effort target, not a guarantee — actual cycle length varies by person, age, and how much sleep debt you're carrying.
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