Definition:
A chemical messenger in the brain that influences sleep and alertness. It is believed to be correlated to the fight-or-flight stress response.
From Sheryl Ankrom, former About.com Guide
Updated October 27, 2008
About.com Health's Disease and Condition content is reviewed by the Medical Review Board
A chemical messenger in the brain that influences sleep and alertness. It is believed to be correlated to the fight-or-flight stress response.