"A state of near-sleep, a strong desire for sleep, or sleeping for unusually long periods. It has two distinct meanings, referring both to the usual
state preceding falling asleep and to the chronic condition that involves being in that state independent of a circadian rhythm."
American Psychiatric Association. Diagnostic and Statistical Manual of Mental Disorders. Fifth edition. Washington, DC. American Psychiatric Association. 2013. Internet. Accessed on June 21, 2017.