What Is Somnolence?

What drowsiness can tell you about your health

Somnolence is a term used to describe the state of sleepiness. It can refer to the drowsiness that naturally occurs as part of the circadian rhythms that direct your sleep/wake patterns. It can also refer to disorders that interfere with circadian rhythm and cause us to be abnormally sleepy, or be associated with certain medications or treatments that cause drowsiness.

Somnolence can be a difficult concept to grasp as it can be either a natural state, a symptom of a disorder, or a disorder unto itself. Within the scope of medicine, however, the term is typically used to describe an abnormal state rather than a normal one.

Somnolence can be broadly classified as being related to one of three things: a physical or mental condition; a medical treatment; or a disorder that misaligns or disrupts the circadian rhythm.

Man with head on desk
Paul Bradbury / OJO Images / Getty Images

Physical and Mental Causes

Sleepiness is a natural response to infection and illness. On the one hand, we sleep because the illness makes us feel run down. On the other, we sleep to conserve energy so that we can get better.

But some conditions directly contribute to somnolence by causing hormonal or chemical balances in the brain. Others affect the brain and nervous system directly, whether it be through injury, infection, or disease. Among the possible causes:

  • Sleep apnea
  • Hypothyroidism (low thyroid hormone production)
  • Hypermagnesemia (too much magnesium)
  • Hyponatremia (too little salt)
  • Hypercalcemia (too much calcium)
  • Meningitis (inflammation of the tissues surrounding the brain and spinal cord)
  • Encephalitis (inflammation of the brain)
  • Brain trauma, including concussion
  • Diabetes
  • Brain tumors
  • Fibromyalgia
  • Bipolar disorders
  • Depression

Treatment-Related Causes

Drowsiness is a common side effect of many prescription and over-the-counter medications. Some of the drugs are specifically used for their tranquilizing effect, while others cause unintended drowsiness due their effect on the central nervous system (CNS).

Non-medication treatments can also cause increased sleepiness due to their effect on the brain. A prime example is radiation therapy used to treat brain cancer. In this case, the use of radiation can trigger a condition known as somnolence syndrome, which is characterized by excessive daytime sleepiness, fatigue, nausea, and vomiting.

One of the main challenges of treatment-related somnolence is that the condition being treated may already be associated with drowsiness. Chief among these are clinical depression and conditions like polycystic ovary syndrome (PCOS), where depression and fatigue are common. In cases like these, a doctor will often change medications or dosages so that the benefits of treatment are not undermined by the side effects.

Some of the classes of drugs most commonly associated with somnolence include:

  • Analgesics (including opiates) used to treat pain
  • Antidepressants
  • Antiepileptics used to treat seizures
  • Antihistamines used to treat allergy
  • Antihypertensives used to treat high blood pressures
  • Antipsychotics
  • Dopamine agonists used to treat conditions like Parkinson’s disease
  • Tranquilizers

Sleep Disorders

Circadian rhythm disorders are those that affect our "internal clock." These sleep abnormalities can either be caused by external (extrinsic) sources or by the internal (intrinsic) malfunction of our sleep/wake patterns.

Extrinsic sleeping disorders are often centered around one key feature: not getting enough sleep at night. The body craves a regular sleep/wake pattern, ideally sleeping at the same time every night and rising at the same time every morning. Any disturbance in this pattern can throw off the circadian rhythm and lead to insomnia and daytime sleepiness.

This relates to experiences like jet lag (caused by time zone changes) and conditions like shift work sleep disorder (SWSD), in which intermittent or rotating shift work can cause a person to slingshot between insomnia and hypersomnia (excessive sleep).

Intrinsic sleeping disorders are not caused by environmental factors or an intentional shift in the sleep routine. Rather, they are associated with a faulty internal clock that causes abnormal sleep/wake patterns. Examples of this include:

  • Advanced sleep phase disorder (ASPD), in which a person gets sleepy and goes to bed early, often before sunset, and rises early, often before sunrise.
  • Delayed sleep phase disorder (DSPD), in which person may not fall asleep until early morning and often sleep through midday.
  • Irregular sleep-wake rhythm disorder, in which a person sleeps intermittently throughout a 24-hour period but has no regular nighttime sleep routine.

Intrinsic disorders are often misdiagnosed as insomnia or hypersomnia rather than a "glitch" in the intuitive sleep/wake cycle. To complicate things even further, no one is quite sure what biological or genetic factors cause these abnormalities.

A Word From Verywell

Daytime sleepiness and drowsiness are problematic for many reasons. They can affect your alertness, mood, and the ability to focus, as well as interfere with your regular sleep patterns at night. If, for example, sleepiness causes you to nap for more than 10 to 15 minutes during the day, you may find that you're suddenly struggling with insomnia at night.

If faced with any sleeping abnormality, it is important to see your doctor so he or she can pinpoint the cause. The solution may be as simple as changing medications, or the evaluation may reveal a medical problem that may have been undiagnosed.

If the sleeping problem is idiopathic (meaning of unknown origin), you will likely need to get a referral to a doctor who specializes in sleep disorders.

Was this page helpful?
Article Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.