"Go to sleep." means what it says. It means to fall asleep supposedly in a bed, but it could be anywhere. However, some people say "Go to sleep." to tell someone to go to bed AND sleep. This is because they assume that someone would sleep in a bed. If you told your friend to go to sleep, they prpbably would not stand upright and sleep on their own two feet whilst standing. They would go to their bed and sleep.

If you tell someone to go to sleep you are usually telling them to fall asleep as they are usually in bed , or someone where they could go to sleep. If you say go to bed they have literally to go to their bed.