What time should I go to bed if I wake up at 12:15 am?

Waking at 12:15 am is a night-shift or early-flight schedule, and the arithmetic does not care: five cycles still means lights out at 4:30 pm, which for most people is the previous afternoon.

Why three times and not one

4:30 pm buys five cycles (7 h 30 min). If you can manage 3:00 pm, that is six cycles and 9 h — the top of the range the NSF panel recommends for adults. The 6:00 pm option is 6 h: enough to get through the day, not enough to do it twice.

Sleep moves from light sleep into deep sleep and then REM, and one pass takes about 90 minutes — anywhere from 70 to 120 depending on the person and the point in the night. That spread is why these are windows, not appointments.

Walk through the night from a 4:30 pm lights-out: asleep by about 4:45 pm, into the first stretch of deep sleep around 5:05 pm, first REM period near 6:00 pm, the halfway point of the night at 8:30 pm, and the longest REM period of the night starting around 11:40 pm — which is what a 12:15 am alarm at the end of cycle 5 lets you finish.

All bedtimes for a 12:15 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 4:30 pm

If 4:30 pm comes and your mind is still running, aim for the next cycle boundary at 6:00 pm instead of counting the minutes. Ten to twenty minutes to fall asleep is normal; much longer, repeatedly, is worth a conversation with a clinician rather than a calculator.

Try a different time