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Classification and external resources
Specialty Sleep medicine
ICD-10 R40.0
ICD-9-CM 780.09
DiseasesDB 16940
MedlinePlus 003208

Somnolence (alternatively "sleepiness" or "drowsiness") is a state of strong desire for sleep, or sleeping for unusually long periods (cf. hypersomnia). It has distinct meanings and causes. It can refer to the usual state preceding falling asleep,[1] the condition of being in a drowsy state due to circadian rhythm disorders, or a symptom of other health problems. It can be accompanied by lethargy, weakness, and lack of mental agility.[2] 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 word "somnolence" is derived from the Latin "somnus" meaning "sleep."


Some features of the human circadian (24-hour) biological clock. Click to enlarge

Circadian rhythm disorders[edit]

Circadian rhythm ("biological clock") disorders are a common cause of drowsiness as are a number of other conditions such as sleep apnea, insomnia, and narcolepsy.[2] The body clock disorders are classified as extrinsic (externally caused) or intrinsic. The former type is, for example, shift work sleep disorder, which affects people who work nights or rotating shifts. The intrinsic types include:[3]

  • Advanced sleep phase disorder (ASPD) – A condition in which patients feel very sleepy and go to bed early in the evening and wake up very early in the morning
  • Delayed sleep phase disorder (DSPD) – Faulty timing of sleep, peak period of alertness, the core body temperature rhythm, hormonal and other daily cycles such that they occur a number of hours late compared to the norm, often misdiagnosed as insomnia
  • Non-24-hour sleep–wake disorder – A faulty body clock and sleep-wake cycle that usually is longer than (rarely shorter than) the normal 24-hour period causing complaints of insomnia and excessive sleepiness
  • Irregular sleep–wake rhythm – Numerous naps throughout the 24-hour period, no main nighttime sleep episode and irregularity from day to day

Physical health[edit]

Sleepiness can also be a response to infection.[4] Such somnolence is one of several sickness behaviors or reactions to infection that some theorize evolved to promote recovery by conserving energy while the body fights the infection using fever and other means.[5][6] Other causes include:[7][8]

Medications that may cause drowsiness[edit]


A number of diagnostic tests, including the Epworth Sleepiness Scale, are available to help ascertain the seriousness and likely causes of abnormal somnolence.[11][12]


Somnolence is a symptom, so the treatment will depend on its cause. If the cause is the behavior and life choices of the patient (like working long hours, smoking, mental state), it may help to get plenty of rest and get distractions. It’s also important to investigate what’s causing the problem, such as stress or anxiety, and take steps to reduce the feeling.[7]

Associated conditions[edit]

See also[edit]


  1. ^ Bereshpolova, Y.; Stoelzel, C. R.; Zhuang, J.; Amitai, Y.; Alonso, J.-M.; Swadlow, H. A. (2011). "Getting Drowsy? Alert/Nonalert Transitions and Visual Thalamocortical Network Dynamics". Journal of Neuroscience 31 (48): 17480–7. doi:10.1523/JNEUROSCI.2262-11.2011. PMID 22131409. 
  2. ^ a b "Drowsiness - Symptoms, Causes, Treatments". Retrieved 2015-10-31. 
  3. ^ "Circadian Sleep Disorders Network". Retrieved 2015-11-02. 
  4. ^ Mullington, Janet; Korth, Carsten; Hermann, Dirk M.; Orth, Armin; Galanos, Chris; Holsboer, Florian; Pollmächer, Thomas (2000). "Dose-dependent effects of endotoxin on human sleep". American Journal of Physiology – Regulatory, Integrative and Comparative Physiology 278 (4): R947–55. PMID 10749783. 
  5. ^ Hart, Benjamin L. (1988). "Biological basis of the behavior of sick animals". Neuroscience & Biobehavioral Reviews 12 (2): 123–37. doi:10.1016/S0149-7634(88)80004-6. PMID 3050629. 
  6. ^ Kelley, Keith W.; Bluthé, Rose-Marie; Dantzer, Robert; Zhou, Jian-Hua; Shen, Wen-Hong; Johnson, Rodney W.; Broussard, Suzanne R. (2003). "Cytokine-induced sickness behavior". Brain, Behavior, and Immunity 17 (1): 112–118. doi:10.1016/S0889-1591(02)00077-6. PMID 12615196. 
  7. ^ a b "Drowsiness: Causes, Treatments & Prevention". Retrieved 2015-10-31. 
  8. ^ "Drowsiness: MedlinePlus Medical Encyclopedia". Retrieved 2015-10-31. 
  9. ^ Zimmermann, C.; Pfeiffer, H. (2007). "Schlafstörungen bei Depression". Der Nervenarzt 78 (1): 21–30. doi:10.1007/s00115-006-2111-1. PMID 16832696. 
  10. ^ Watanabe, Norio; Omori, Ichiro M; Nakagawa, Atsuo; Cipriani, Andrea; Barbui, Corrado; Churchill, Rachel; Furukawa, Toshi A (2011). "Mirtazapine versus other antidepressive agents for depression". Cochrane Database of Systematic Reviews (12): CD006528. doi:10.1002/14651858.CD006528.pub2. PMC 4158430. PMID 22161405. 
  11. ^ Carskadon, M.A.; Dement, W.C.; Mitler, M.M.; Roth, T.; Westbrook, P.R.; Keenan, S. Guidelines for the Multiple Sleep Latency Test (MSLT): a standard measure of sleepiness. Sleep 1986; 9:519–524
  12. ^ Johns, MW (March 2000). "Sensitivity and specificity of the multiple sleep latency test (MSLT), the maintenance of wakefulness test and the epworth sleepiness scale: failure of the MSLT as a gold standard". Journal of Sleep Research 9 (1): 5–11. doi:10.1046/j.1365-2869.2000.00177.x. PMID 10733683.