Sleeping is a way for our brains to flash out all the toxins that accumulated in the brain throughout the day. A kind of human garbage collector.
I never limit my sleep and I try to sleep as much as my body needs thus I wake up with no alarm.
I try to go to bed at the same time each day to maintain the Circadian rhythm. I get daily reminder at 22:20 to check my habits and prepare to go to sleep (~ 22:30).