What time do you go to bed? What time do you usually get up? How much time do you have for sleeping?
I heard that some people (like Elon Musk) sleep for 4 hours in order to have more time to do things during the day. Is it dangerous to health?
Yes, this is very dangerous for your health. The body and brain must at least get about 7 hours of sleep on a regular basis, if not this can eventually lead to health consequences that affect your entire body. Constant lack of sleep will slow down your brain, these supposedly saved hours so they can work longer in the day will not help you in any way, because your productivity will be much lower than the usual way of working.
I try to sleep at least 7 hours a day or else I'll get sleep deprivation and reduce my quality of sleep.