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. 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 as a symptom of 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. "Somnolence" is derived from the Latin "somnus" meaning "sleep."


  • Causes 1
    • Circadian rhythm disorders 1.1
    • Other causes 1.2
    • Medications that may cause drowsiness 1.3
  • Diagnosis 2
  • Treatment 3
  • Associated conditions 4
  • See also 5
  • References 6


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

Circadian rhythm disorders

Circadian rhythm disorders are a common cause of drowsiness as are a number of other conditions such as sleep apnea, insomnia, and narcolepsy.[2]

Other causes

Sleepiness can also be a response to infection.[3] 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.[4][5] Other causes include:[6][7]

Medications that may cause drowsiness


A number of diagnostic tests, including the Epworth Sleepiness Scale, are available to help ascertain the seriousness and likely causes of abnormal somnolence.[10][11]


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.[6]

Associated conditions

See also


  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.  
  2. ^ a b "Drowsiness - Symptoms, Causes, Treatments". Retrieved 2015-10-31. 
  3. ^ 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.  
  4. ^ Hart, Benjamin L. (1988). "Biological basis of the behavior of sick animals". Neuroscience & Biobehavioral Reviews 12 (2): 123–37.  
  5. ^ 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.  
  6. ^ a b "Drowsiness: Causes, Treatments & Prevention". Retrieved 2015-10-31. 
  7. ^ "Drowsiness: MedlinePlus Medical Encyclopedia". Retrieved 2015-10-31. 
  8. ^ Zimmermann, C.; Pfeiffer, H. (2007). "Schlafstörungen bei Depression". Der Nervenarzt 78 (1): 21–30.  
  9. ^ 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.  
  10. ^ 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
  11. ^ 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.