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.
By Sheryl Ankrom, About.com
Updated: October 26, 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.
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