Each night your brain moves through several sleep cycles.
At the start of each cycle you enter light sleep,
then move into deep sleep, then dream,
and finally move back into light sleep.
These cycles take about 90 minutes each,
and you will feel most refreshed
if you wake up at the end of a cycle
because then you are closest to your normal waking state.
To increase the chances of this,
decide when you want to wake up
then count back in 90-minute blocks
until you find a time close to when want you to go to sleep.
So let’s imagine that you want to wake at 8 a.m
Chunking back in ninety-minute blocks would look like this:
and so you should fall asleep around either eleven o’clock, or half past twelve, at night.
And so if you want to wake up feeling especially alert and refreshed,
simply follow the 90 minute rule.