The changes in quantity of a certain neurotransmitter as well as how the post-synaptic terminal responds to this change are underlying mechanisms of brain plasticity.[27] During sleep there are remarkable changes in modulatory neurotransmitters throughout the brain.Acetylcholine is an excitatory neurotransmitter that is seen to increase to near waking levels during REM sleep while compared to lower levels during slow-wave sleep.[36] Evidence has shown that functioning of the hippocampus dependent memory system (episodic memory and autobiographical memory) is directly affected by cholinergic changes throughout the wake-sleep cycle. High levels of ACh would promote information attained during wakefulness to be stored in the hippocampus. This is accomplished by suppressing previous excitatory connections while facilitating encoding without interference from previously stored information.