The parasympathetic nervous system controls rest, digestion, and sleep.

“The parasympathetic nervous system is often referred to as the 'rest and digest system'. And indeed, it has roles in rest and digestion, but it controls a lot more than just that. The parasympathetic branch of the autonomic nervous system controls, for instance, digestion. It controls our ability to fall asleep at night. If the parasympathetic nervous system is overly activated, it can make us sleepy when we don't want to be sleepy. It can make us pass out when we don't want to pass out. It can be responsible for putting people into a state of coma.”