I have read in a book long time ago, the sleep its done in circles of 1:30 hours each. First 40-50 minutes, its that sleep when you dont have dreams, when you can take a moment to rest yourself during the day and you dont wake up "drunk".

After 40-50 minutes, the circles start to be active, here the deep sleep take place, you start have dreams, brain activity, eyes moving and so on.

The most important thing to this circles its to know when you wake up, for example if you wake up between them its very rough for organism to recover, the day its a mess and you are tired entire day.

A good sleep for me based on experience and knowing this its about 7 hours.

If you start put on paper to see when you wake up without alarm, you will see you wake up after a circle its done. Not between them.