Work backward from your wake time to a bedtime that lands your alarm at the end of a sleep cycle.
If you know when you need to wake up, the better question isn't “how many hours?” — it's
“which bedtime lands my alarm at the end of a sleep cycle?” Here's the fast version.
The 15 + 90 method
Start from your fixed wake time.
Subtract 15 minutes (time to fall asleep).
Subtract 90-minute cycles — aim for 6 (9 hrs) or 5 (7.5 hrs).
If you wake at
Go to bed at (6 or 5 cycles)
5:30 AM
9:00 / 10:30 PM
6:00 AM
9:30 / 11:00 PM
6:30 AM
10:00 / 11:30 PM
7:00 AM
10:30 PM / 12:00 AM
7:30 AM
11:00 PM / 12:30 AM
8:00 AM
11:30 PM / 1:00 AM
Most adults feel best at 5–6 cycles. If you're consistently exhausted at 6, you may simply need more
total sleep — check the by-age chart.
Make the bedtime stick
Keep the same wake time every day — weekends included. It's the single biggest lever on sleep quality.
Dim lights and stop caffeine ~6 hours before bed.
If you're not asleep in ~20 minutes, get up, do something calm and dim, and return when sleepy.