Somnolence (alternatively "sleepiness" or "drowsiness") is a state of strong desire for sleep, or sleeping for unusually long periods (compare 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]

Other namesSleepiness, drowsiness.

Somnolence is often viewed as a symptom rather than a disorder by itself. However, the concept of somnolence recurring at certain times for certain reasons constitutes various disorders, such as excessive daytime sleepiness, shift work sleep disorder, and others; and there are medical codes for somnolence as viewed as a disorder.

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. In individuals deprived of sleep, somnolence may spontaneously dissipate for short periods of time; this phenomenon is the second wind, and results from the normal cycling of the circadian rhythm interfering with the processes the body carries out to prepare itself to rest.

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

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

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/sleepiness

Assessment of Pathological Sleepiness

Quantifying sleepiness requires a careful assessment. The diagnosis depends on two factors, namely chronicity and reversibility. Chronicity signifies that the patient, unlike healthy people, experiences persistent sleepiness, which does not pass. Reversibility stands for the fact that even if the individual goes to sleep, the sleepiness may not be completely gone after waking up. The problem with the assessment is that patients may only report the consequences of sleepiness: loss of energy, fatigue, weariness, difficulty remembering or concentrating, etc. It is crucial to aim for objective measures to quantify the sleepiness. A good measurement tool is the Multiple Sleep Latency Test (MSLT). It assesses the sleep onset latency during the course of one day - often from 8:00 to 16:00.[11] An average sleep onset latency of less than 5 minutes is an indication of pathological sleepiness.[12]


A number of diagnostic tests, including the Epworth Sleepiness Scale, are available to help ascertain the seriousness and likely causes of abnormal somnolence.[13][14]


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 rid of 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]

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. doi:10.1523/JNEUROSCI.2262-11.2011. PMC 6623815. PMID 22131409.
  2. "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. doi:10.1152/ajpregu.2000.278.4.r947. 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. "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. Kushida, Clete A.; Sullivan, Shannon S. (2008-10-01). "Multiple Sleep Latency Test and Maintenance of Wakefulness Test". Chest. 134 (4): 854–861. doi:10.1378/chest.08-0822. ISSN 0012-3692. PMID 18842919.
  12. Roehrs, Timothy; Carskadon, Mary A.; Dement, William C.; Roth, Thomas (2017), "Daytime Sleepiness and Alertness", Principles and Practice of Sleep Medicine, Elsevier, pp. 39–48.e4, doi:10.1016/b978-0-323-24288-2.00004-0, ISBN 9780323242882
  13. 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
  14. 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. Archived from the original on 2012-12-10.
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