Sleepiness vs. Fatigue: What Is the Difference?

Man resting on the job
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It may seem like semantics, arguing over the meanings of similar words, but the difference between sleepiness and fatigue really does matter. Not only can discriminating between these distinctive feelings identify different causes, but it may also help to treat certain disorders.

Sleepiness

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're sitting after lunch in your most comfortable chair. You're cozy and relaxed. Your eyelids become heavy, and each time they close, they stay that way a moment longer. You are ready to doze off. You're 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's a signal that we need sleep.

Since the levels of adenosine build up all day, the strongest drive for sleep occurs at the end of the day. 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's no wonder that people fall asleep watching TV or reading right before their regular bedtime.)

In turn, sleepiness is relieved by sleep itself. If you get enough hours of normal quality sleep, you wake feeling refreshed and 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're 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. But, no matter how extreme the fatigue, it doesn't result in sleep.

People who feel fatigued may lie down to rest or take a nap. However, they often don't fall asleep (though people with extreme sleepiness or drowsiness will be able to sleep if given the opportunity). Moreover, this sense of fatigue may not even be relieved by sleep.

Deprivation and 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.

Insomnia

Not only does distinguishing between sleepiness and fatigue lead to a different set of possible causes to your problem, but recognizing sleepiness can also contribute to improving insomnia. 

It's critically important for people to only go to bed when they feel sleepy. If fatigue (or worse, time of 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 evening, 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 your onset of sleep. It's counterintuitive, but effective. By staying up later, the desire for sleep builds. Instead of going to bed at 9 p.m., if you have insomnia, you may be advised to stay up until midnight. If you keep your wake time fixed to 6 a.m., the sleep period becomes consolidated and it becomes easier to fall asleep.

In addition, the quality and depth of sleep are 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.

A Word From Verywell

Consider carefully whether you're 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.

If you continue to suffer from sleep that is either insufficient due to poor quality or from too few hours of sleep, seek help from a board-certified sleep physician. It may be necessary to explore the condition with a sleep study.

In some cases, your lack of sleep 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. 

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Article Sources
  • Kryger MH, Roth T,  Dement WC. Principles and Practice of Sleep Medicine. Elsevier, 6th edition; 2017.