|Other names||Sleepiness, drowsiness.|
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, 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.
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".
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. 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:
- 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
Sleepiness can also be a response to infection. 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. Other causes include:
- Brain tumor
- Chronic pains
- Concussion – a mild traumatic brain injury
- Head injury
- Hypercalcemia – too much calcium in the blood
- Hyponatremia – low blood sodium
- Hypothyroidism – the body doesn't produce enough hormones that control how cells use energy
- Mood disorders – depression
- Narcolepsy – disorder of the nervous system
- Skull fractures
- Sleeping sickness – caused by a specific parasite
Medications that may cause drowsiness
- Analgesics – mostly prescribed or illicit opiates such as OxyContin or heroin
- Anticonvulsants / antiepileptics – such as phenytoin (Dilantin), carbamazepine (Tegretol), Lyrica (Pregabalin) and 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, Nytol) and doxylamine (Unisom-2)
- Antipsychotics – for example, thioridazine, quetiapine (Seroquel), olanzapine (Zyprexa), risperidone, and ziprasidone (Geodon) but not haloperidol
- Dopamine agonists used in the treatment of Parkinson's disease – e.g. pergolide, ropinirole and pramipexole.
- HIV medications – such as efavirenz
- Hypertension medications – such as amlodipine
- Tranquilizers / hypnotics – such as zopiclone (Zimovane), or the benzodiazepines such as diazepam (Valium) 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
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.
Effect of somnolence on electroencephalography
In healthy subjects, electroencephalography (EEG) shows distinct characteristics correlated to the level of somnolence experienced by a subject. To better measure these changes, and thus the level of somnolence, a common approach is to analyze the low frequency band (for example delta, alpha and theta waves) of the electroencephalographic signal. EEG frequency analysis is a standard approach that can provide insights into sleep and waking brain activity. While being in a drowsy state, the EEG shows higher slow frequency activity than compared to a rested waking condition. These changes may be used to track the somnolence state of a subject using only a handful of electrodes applied on the scalp and paired with an EEG recorder. For example, this very same technique has been used during long haul flights in aviation pilots to objectively measure their somnolence during the flight. Similar approaches are used also for car drivers somnolence detection.
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