Learn the Difference Between Sleepiness and Fatigue

The difference between sleepiness and fatigue may depend on levels of the chemical adenosine within the brain
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It may seem like semantics, arguing over the meanings of similar words, but it really does matter: What is the difference between feeling sleepiness and fatigue? Not only may discriminating between these distinctive feelings identify different causes, but it may also help to treat insomnia.

Feeling Sleepy

Some people lose touch with what it feels like to be sleepy. Sleepiness or drowsiness is the extreme desire to fall asleep.

Imagine that you are sitting after lunch in your most comfortable chair. You are cozy and relaxed. Your eyelids become heavy, each time they close they stay that way a moment longer. You are ready to doze off. You are sleepy.

Generally, feelings of sleepiness build the longer a person stays awake. This has to do with the build up of a chemical in the brain called adenosine. It is a signal for sleep. Therefore, the strongest drive for sleep occurs at the end of the day. The levels of adenosine have typically built for up to 16 hours. As a result, most people feel sleepy in the evening, with an overwhelming desire for sleep at its highest peak right before the onset of sleep. It is no wonder that people fall asleep watching TV or reading right before their regular bedtime.

Sleepiness is relieved by sleep. If you get enough hours of normal quality sleep, you wake feeling refreshed. The desire for sleep should be almost fully diminished upon awakening.

Fatigue and Exhaustion

Contrast this sleepiness with a different collection of words: fatigue, tiredness, exhaustion, and low energy. These sentiments are felt deep in the bones and muscles, a heaviness to the limbs, as if you just ran a marathon. You can't summon the energy to accomplish what you need to.

You are physically and mentally dragging through the day. This may occur in the setting of other illness, such as anemia, hypothyroidism, or even cancer. It may even be labeled as chronic fatigue syndrome.

No matter how extreme the fatigue, it does not result in sleep. People who feel fatigued may lie down to rest or take a nap. They often do not, however, fall asleep. (People with extreme sleepiness or drowsiness will be able to sleep if given the opportunity.) Moreover, this sense of fatigue may not be relieved by sleep. Why does this matter?

Sleep Deprivation and Sleep Disorders

Sleepiness often occurs in sleep deprivation among those who get inadequate total sleep time. It may also be a symptom of sleep disorders such as sleep apnea or narcolepsy. In contrast, fatigue is a common complaint among those with insomnia.

Not only does distinguishing between sleepiness and fatigue lead to a different set of possible causes, but recognizing sleepiness can also contribute to improving insomnia. How might this work? It is critically important for people to only go to bed when they feel sleepy. If fatigue (or worse still simply the time of the night) is used as a prompt to go to bed, this may result in lying awake for prolonged periods of time at the start of the night, trying to fall asleep.

As anxiety builds, it further overrides the signal for sleepiness. This is a major contributor to insomnia.

One of the most effective remedies for insomnia is to delay the onset of sleep. It is counterintuitive, but effective. By staying up later, the desire for sleep builds. Instead of going to bed at 9 PM, the person with insomnia may be advised to stay up until midnight. If this person keeps the wake time fixed to 6 AM, the sleep period becomes consolidated. It becomes easier to fall asleep. In addition, the quality an depth of sleep is enhanced. After an initial period of sleep restriction, the time spent in bed can be extended incrementally so that adequate hours of rest are obtained.


Consider carefully whether you are having more difficulty with sleepiness or fatigue. It may point to a distinct underlying cause and correcting it will depend on a different set of treatments. As you work to sleep better, reflect on your own needs and familiarize yourself with the feeling of sleepiness.

A Word From Verywell

If you continue to suffer from sleep that is either insufficient due to poor quality, or from too few hours, seek help from a board-certified sleep physician. It may be necessary to explore the condition with a sleep study. In some cases, the insomnia may relent with cognitive behavioral therapy for insomnia (CBTI), a guided 6-week program that optimizes sleep. CBTI can be provided by a sleep psychologist or through participation in a workshop or online course. 


Kryger, MH et al. Principles and Practice of Sleep Medicine. Elsevier, 6th edition, 2017.

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