Somnolence (or "sleepiness") is a state of near-sleep, a strong desire for sleep, or sleeping for unusually long periods (cf. hypersomnia). It has two distinct meanings, referring both to the usual state preceding falling asleep, and the chronic condition referring to being in that state independent of a circadian rhythm. "Somnolence" is derived from the Latin "somnus" meaning "sleep."
Sleepiness can be dangerous when performing tasks that require constant concentration, such as driving a vehicle. When a person is sufficiently fatigued, microsleeps may be experienced.
The human body can become sleepy in response to infection. Such somnolence is one of several sickness behaviors or reactions to infection that some theorize evolved due to promoting recovery by conserving energy while the body fights the infection using fever and other means.
A number of diagnostic tests, including the Epworth Sleepiness Scale, are available to help ascertain the seriousness and likely causes of abnormal somnolence.
^Bereshpolova Y, Stoelzel CR, Zhuang J, Amitai Y, Alonso JM, Swadlow HA. (2011) "Getting drowsy? Alert/nonalert transitions and visual thalamocortical network dynamics". J Neurosci. 2011, 48: 17480-17487. PubMed 22131409
^Mullington, J.; Korth, C.; Hermann, D. M.; Orth, A.; Galanos, C.; Holsboer, F.; Pollmächer, T. (2000). "Dose-dependent effects of endotoxin on human sleep". American journal of physiology. Regulatory, integrative and comparative physiology278 (4): R947–R955. PMID10749783. edit