Definition and Possible Causes of Sleepiness

It may seem like an odd thing to consider, but what is sleepiness and what causes it? Does sleepiness suggest the presence of certain sleep disorders like sleep apnea or narcolepsy? And how does sleepiness differ from fatigue or tiredness? Let's find out.

Synapse in the brain
cosmin4000 / Getty Images

What Is Sleepiness?

Sleepiness is the desire to fall asleep. It is sometimes referred to as drowsiness and typically increases the longer we stay awake. To understand what is really meant by sleepiness, consider this extreme example:

Imagine that you didn’t sleep enough over the last several nights and you then had a delicious lunch with turkey, mashed potatoes and gravy, and plenty of alcohol. It’s now two in the afternoon and you're sitting comfortably in a big soft chair in a room that's too warm. You're reading a boring book or listening to an uninteresting television program. Your eyelids are getting heavy. A warm feeling comes over you. You are about to doze off. You are sleepy.

More specifically, sleepiness relates to the accumulation of a chemical messenger, or neurotransmitter, within the brain called adenosine. Adenosine may build up between and within nerve cells and higher levels of it in the reticular activating system of the brainstem are associated with greater levels of sleepiness. (Interestingly, caffeine works by blocking the action of adenosine within the brain, leading to wakefulness. Alcohol enhances it and contributes to feeling sleepy.) Sleepiness may occur normally or be due to an underlying sleep disorder.

People feel sleepy every day, especially before sleep onset. The degree of sleepiness or drowsiness may increase the longer you stay awake. You'll also feel more sleepy during times that you are meant to be asleep, such as during the night. This relates to the role of the circadian rhythm. Sleepiness may also be worse during periods of sleep deprivation. If poor quality sleep is obtained, you may feel more sleepy during the day.

Possible Causes

Too much sleepiness with an adequate amount of time spent sleeping may suggest a sleep disorder. Excessive daytime sleepiness, often measured by the Epworth sleepiness scale, is a common complaint among those with sleep apnea or narcolepsy. Each of these conditions leads to sleep fragmentation, which disrupts the restorative process of sleep. Some people have sleepiness without a clear cause as a result of a condition called idiopathic hypersomnia.

Additionally, sleepiness may result from consuming foods rich in tryptophan (like turkey), drinking alcohol, or as a side effect to medications (including sleeping pills). After eating a meal, feeling drowsy is called post-prandial sleepiness.

Sleepiness or Fatigue?

It's important to differentiate sleepiness or drowsiness from fatigue or tiredness. In the realm of sleep problems, fatigue is a common complaint of insomnia. People with fatigue often feel tired, but if given the opportunity, they are usually unable to take a nap. There are some ways to avoid sleepiness.

A Word From Verywell

If you're struggling with excessive sleepiness, you should speak with your healthcare provider to determine if you may have an underlying sleep disorder.

6 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.
  1. Holst SC, Landolt H-P. Sleep homeostasis, metabolism, and adenosineCurr Sleep Medicine Rep. 2015;1(1):27-37. doi:10.1007/s40675-014-0007-3

  2. Ferré S, O'Brien MC. Alcohol and caffeine: the perfect storm. J Caffeine Res. 2011;1(3):153-162. doi:10.1089/jcr.2011.0017

  3. National Institute of General Medical Sciences. Circadian rhythms.

  4. Slater G, Steier J. Excessive daytime sleepiness in sleep disorders. J Thorac Dis. 2012;4(6):608-616. doi:10.3978/j.issn.2072-1439.2012.10.07

  5. Lieberman HR, Agarwal S, Fulgoni VL. Tryptophan intake in the US adult population is not related to liver or kidney function but is associated with depression and sleep outcomes. J Nutr. 2016;146(12):2609S-2615S. doi:10.3945/jn.115.226969

  6. Popp RF, Fierlbeck AK, Knüttel H, et al. Daytime sleepiness versus fatigue in patients with multiple sclerosis: A systematic review on the Epworth sleepiness scale as an assessment tool. Sleep Med Rev. 2017;32:95-108. doi:10.1016/j.smrv.2016.03.004

By Brandon Peters, MD
Brandon Peters, MD, is a board-certified neurologist and sleep medicine specialist.