Lucid dreaming takes place in REM sleep. It is characterized by a higher amount of CNS (central nervous system) activity than normal, non-lucid REM sleep. Normally in REM sleep the critical faculty of the brain is switched off. More specifically, the aminergic neurons in the brainstem are at their lowest levels in REM sleep. The neurotransmitters responsible for activating the aminergic neurons in the brainstem are somehow inhibited, or more accurately, being conserved for the following day.