It is about your regular routine that carried forward.

I also sleep around 11:00 ~ 11:30 pm but wake up at 4:00am every morning, even during weekend. Even though alarm is set at 4am, I usually wake up before the alarm. I do not start work at 6am but around 6:40am I'm already out to work. Work finish at 5:45pm.

What I meant was I do not start work at 6am like you. I go out to work at 6.40am and reaches office around 7:30~7:40am. Work starts at 8:00am~5:45pm

Your body will adapt sooner or later and become routine. No need alarm clock. How much rest you body needed depends on your work or how exhausted you are. If more physical then probably longer resting hour. I mentioned last time I sleep around 11.00 to 11.30pm and still wake up at 4am. Coming back from work if traffic is bad will reach around 7pm.

When I worked in the US, the work hour is flexible. you just need 8 hour working time. I started at 5am and went back at 2pm. No traffic jam Many technicians work at that time. Well admin people has to follow office hour though