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Somnolence (or "drowsiness") 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.
- advanced sleep phase disorder
- Alice in Wonderland syndrome
- brain edema
- cerebral hypoxia
- chronic fatigue syndrome
- clinical depression, especially seasonal affective disorder (SAD)
- delayed sleep phase syndrome
- diabetes – ketoacidosis as example, but not balanced diabetes mellitus
- encephalitis – (viral, bacterial or other agents)
- epilepsy – after seizure
- idiopathic hypersomnia
- infectious mononucleosis (glandular fever)
- intracranial hemorrhage such as due to ruptured aneurysm
- increased intracranial pressure; for example, due to brain tumors
- lyme disease (borreliosis)
- analgesics – mostly prescribed or illicit opiates such as OxyContin or heroin
- anticonvulsants / antiepileptics – such as phenytoin (Dilantin), carbamazepine (Tegretol), Lyrica (pregbalin), Gabapentin
- antidepressants – for instance sedating tricyclic antidepressants, and mirtazapine. Somnolence is less common with SSRIs and SNRIs as well as MAOIs.
- antihistamines – for instance, diphenhydramine (Benadryl) and doxylamine (Unisom-2)
- antipsychotics – for example, thioridazine, quetiapine, olanzapine (Zyprexa), risperidone, and Geodon but not haloperidol
- dopamine agonists used in the treatment of Parkinson's disease – e.g. pergolide, ropinirole and pramipexole.
- HIV medications – for example, Sustiva and efavirenz
- hypertension medications – such as amlodipine
- tranquilizers / hypnotics – such as zopiclone, or the benzodiazepines such as diazepam or nitrazepam (Mogadon) and the barbiturates, such as amobarbital (Amytal) or secobarbital (Seconal)
- other agents impacting the central nervous system in sufficient or toxic doses
- sickness behavior
- sleep apnea
- sleep deprivation / insomnia
- traumatic brain injury
- trypanosomiasis ("sleeping sickness")
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