What time should I go to bed if I wake up at 6:15 am?

Set for 6:15 am? Lights out at 10:30 pm gives five cycles (7 h 30 min); 9:00 pm gives six (9 h).

Why three times and not one

The middle option, 10:30 pm, is five cycles and 7 h 30 min of actual sleep; 9:00 pm is six cycles and 9 h. Both sit inside the adult range. The 12:00 am option is 6 h: enough to get through the day, not enough to do it twice.

Why cycles at all? Because what you wake from matters more than how long you slept: an alarm in deep sleep leaves ten to thirty minutes of fog, an alarm in light sleep does not. Cycles average 90 minutes but range from 70 to 120, so aim for the window rather than the minute.

Here is the shape of the night. You settle at 10:30 pm and are asleep by 10:45 pm; the first and deepest slow-wave stretch arrives near 11:05 pm; by 2:30 am you are halfway through; from about 5:40 am the final REM period runs up to the 6:15 am alarm. Cut the night short and that last REM block is what goes.

All bedtimes for a 6:15 am alarm

Formula: bedtime = wake time − (cycles × 90 min) − 15 min · adult range from the NSF 2015 panel and the AASM/SRS 2015 consensus · methodology

If you can't fall asleep by 10:30 pm

Fifteen minutes is the average time a healthy sleeper takes to drop off; ten to twenty is normal. If you are still awake twenty minutes after 10:30 pm, sleep-medicine guidance is to get up, do something dull in dim light, and return when sleepy — lying there teaches the brain that bed is for being awake. Miss the window and the next one, 12:00 am, is only a cycle away.

Try a different time