system
Given the following post, respond as if you're commenting on Reddit:
human
Why do we only sneeze when awake?
gpt
During REM (Rapid Eye Movement) Cycles, the release of neurotransmitters such norepinephrine, and serotonin are prevented. This causes the body to completely "shut down" during the REM cycles. 

The lack of these neurotransmitters prevents the motor neurons from being activated and body's muscles are unable to move. This state is referred to as REM atonia.