Sleep is not a single state in which we remain while we sleep, but has 5 stages. During the course of the night, in the dream, we go through each of these stages, completing them all in a cycle that lasts approximately 90 minutes. Stages 1 and 2 are considered to be “light” sleep, while stages 3 and 4 are considered “deep” sleep. The fifth stage, which is completely different from the rest, is the phase of REM sleep (rapid eye movements), which is when we dream.
Research has shown that the duration of sleep it’s not what makes us feel fresh and clear. The key factor is the number of complete cycles we do. Every cycle of the dream, like we have already said, lasts approximately 90 minutes: 65 minutes of normal sleep (or non-REM, the first 4 stages); 20 minutes of REM sleep (the fifth stage); and 5 ending minutes in a non-REM state.
In the period between 2 sleep cycles we are not really sleeping: it is a kind of intermediate zone, in which if we are not disturbed (by the light, the cold, the noise, etc.), we enter another cycle of 90 minutes. It is a form that the brain has to be half alert to possible dangers, that is, you do not completely deactivate when you sleep, but you do it in cycles to avoid any possible danger.
For this reason, if you manage to wake up between cycles, even though you may be sleeping less hours than necessary, you may feel better by respecting these 90 minutes than if you sleep more than you count, but you wake up or wake you up in half of some of the cycles. That is to say if you sleep 4 hours and a half (multiple of 90 minutes) you will wake up much more rested than if you do it after 8 hours.
So the secret is obvious, sleep cycles of 90 minutes also taking into account what hard to sleep. An example, if you want to wake up at 8 in the morning and it is 22 at night you should sleep at 23:00 for 6 complete cycles of sleep, 00:30 for 5 cycles, 02:00 for 4 cycles, etc. Attentive to the fact that I said sleep, that is, you will have to go to bed 5-15 minutes in advance so that you are more or less asleep at the chosen time.
If you are going to take a nap or if you want to sleep a bit although it is too late consider this detail and you will feel much better when you wake up. Remember that you should try to sleep about 5-6 cycles, and I also have observed that before midnight if you start the rest period goes much better. I have noticed the difference.