Why do hands go to sleep when I'm sleeping every night?
It's not circulation of blood that's missing. It's pressure on certain nerves in your arm . When you change position the pressure gets relieved and sensation returns.
Because your brain is asleep :)
If you mean the pins and needles feeling, you could be laying on them and stopping circulation.
Hands go to sleep when you are sleeping because your brain is not working, to direct them. Hands work according to the directions they get from the brain. That is why you cannot do anything, unless you are fully awake.