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Most people pick a bedtime by tiredness, which is exactly backwards — your wake time is the anchor, and your bedtime should be built from it. Sleep runs in roughly 90-minute cycles, and waking mid-cycle is why you can sleep eight hours and still feel foggy. This calculator counts back from the time you must wake and shows your natural cycle-end bedtimes, so mornings stop feeling like an ambush.
Work backwards from your wake time using 90-minute sleep cycles to find a bedtime that leaves you waking at a cycle end — less grogginess, better mornings.
Sleep alternates through ~90-minute cycles; waking at a cycle's end leaves you far more alert than waking mid-cycle, even with the same total sleep.
No — adults need 7-9 hours. Count back 4-6 cycles (6-9 hours) from your wake time and test which number leaves you refreshed.
Always keep the wake time fixed and move the bedtime. A stable wake time anchors your body clock more than any bedtime.
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