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 feel most refreshed if you wake up at the end of the cycle, because by 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 you want to fall asleep.

So let’s imagine you want to wake up at 8 AM.

Chunking back in 90-minute blocks would look like this.

And so you should aim to fall asleep around either 11 o’clock, or half past 12 at night.

And so if you want to wake up feeling especially alert and refreshed, simply follow the 90-minute rule.