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 meant by sleepiness, consider an 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 2 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.

The Cause of Sleepiness
Sleepiness relates to the accumulation of a chemical within the brain called adenosine. This messenger (or neurotransmitter) may build up between and within the nerve cells. Higher levels of adenosine 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.

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 in 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.

How Fatigue Differs from Sleepiness
It's important to differentiate sleepiness or drowsiness from fatigue or tiredness. Fatigue may represent a separate set of medical conditions, including anemia, hypothyroidism, and depression. 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 often unable to take a nap. If you're struggling with excessive sleepiness, you should speak with your doctor to determine if you may have an underlying sleep disorder contributing to your complaint.