What time should I go to bed if I wake up at 8:00 pm?

For a 8:00 pm alarm the day is your night. Five cycles start at 12:15 pm; the minimum four start at 1:45 pm.

Why three times and not one

The middle option, 12:15 pm, is five cycles and 7 h 30 min of actual sleep; 10:45 am is six cycles and 9 h. Both sit inside the adult range. The 1:45 pm option is 6 h: enough to get through the day, not enough to do it twice.

Each cycle runs light sleep, deep sleep, then REM. Ninety minutes is the average; the real figure drifts between 70 and 120 minutes, and the first cycle of the night is usually the shortest.

Walk through the night from a 12:15 pm lights-out: asleep by about 12:30 pm, into the first stretch of deep sleep around 12:50 pm, first REM period near 1:45 pm, the halfway point of the night at 4:15 pm, and the longest REM period of the night starting around 7:25 pm — which is what a 8:00 pm alarm at the end of cycle 5 lets you finish.

All bedtimes for a 8:00 pm alarm

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

8:00 pm: staying up through the morning first

An 8:00 pm alarm means the main sleep starts around midday: 12:15 pm for five cycles, 10:45 am for six, 1:45 pm for four. That is a deliberate pattern more than a late one. You come off a night shift, stay up through the morning for errands, appointments or daylight, and sleep in the afternoon so that waking is close to the next shift. With an 11 pm start you are three hours from work when the alarm goes, and by 7 am you have been awake 11 hours, six fewer than with a 2:00 pm alarm.

What it asks of you is the morning. Clock off at 7 am, go to bed at 12:15 pm, and you have been up since 8:00 pm the evening before by the time you lie down, with the last of those hours spent after a full shift. If the errands involve a car, do them early or leave them for a day off. Coffee is the other trap: the six-hour line for a 12:15 pm bedtime is 6:15 am, so the cup that gets you through the school run is one the 2013 timing trial would count against your sleep. The afternoon dip in alertness works in your favour, though. Falling asleep just after noon is usually easier than at 8 am, when a body on any schedule is gearing up for the day.

Every alarm in the 8 pm hour

Lights-out times for each alarm · last coffee = six hours before the five-cycle bedtime (Drake 2013) · full grid on the open data page

If you can't fall asleep by 12:15 pm

Build in the fifteen minutes honestly: lights out at 12:15 pm means phone down at 12:15 pm, not "in bed, scrolling". If sleep has not come by 12:35 pm, the guidance from sleep medicine is to leave the bed, do something dull in dim light, and return when sleepy — the next clean boundary is 1:45 pm.

Try a different time