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 and wake up at the same time each day to maintain the Circadian rhythm. I try to go to sleep at ~ 23:00 every day for consistency.