Fatigue connotes a longer time period than sleepiness. While sleepiness can generally go away after a few hours of sleep, fatigue may take longer to recover from. There is even "chronic fatigue syndrome". Some distinguish between muscle or physical fatigue and mental fatigue. The dividing line here is unclear because our mental lives derive from physical processes.
Both overstimulation and understimulation can cause mental fatigue. It is often a diagnostic challenge to figure out what is going on insider a person. A survey showed women with obstructive apnea are more likely to report the symptom of fatigue while men are more likely to report sleepiness. A Swedish study found glazed eyes was a characteristic significantly related to looking fatigued but not to sleep deprivation. They also found fatigued people were rated by observers as appearing sad.
We normally don’t think of fatigue or sleepiness as emotions, but it is possible to see it that way: fatigue as a motivational drive to rest. In the 19th Century an Italian physiologist praised fatigue as part of the body’s homeostatic system. If we consider fatigue a brain-derived emotion (moreso that a mind-driven emotion), we can see both the skeletal muscles and the nervous system change in response to extended exercise, to prevent excessive wear on the body. If fatigue is an emotion – a "conscious sensation rather than a physiological occurrence" - it could reflect the mind’s awareness of the brain’s physiology. As a complex emotion fatigue would be caught up in and affected by fear, anger, motivation, etc. Scientists don’t know whether the mechanisms that the conscious mind perceives the fatigue localized to one or a few sections of the brain, or to the whole brain.
There is a difference between fatigue and sleepiness. Fatigue, sometimes called tiredness, can result from overwork as well as inactivity and unhealthy eating. There are also people who have fatigue from a variety of medical situations (diseases, chemotherapy treatment, etc.) Sleepiness often accompanies fatigue, but they are not the same. The way to recover from fatigue is to rest. The way to recover from sleepiness is to sleep. You can rest without sleeping. Chemicals or drugs like stimulants can counteract both fatigue and sleepiness for a short time.
Ergonomics experts worry about both fatigue and sleepiness on the job as they often occur together and can both contribute to risk of accident or injury.
One problem if you don’t distinguish between fatigue and sleepiness: responding to the wrong signals. People with a history of insomnia might find themselves fatigued and go to bed. They lie awake in frustration. But they aren’t really sleepy, they are fatigued. People attempting to do sleep restriction therapy would do well to be aware of the difference. If you want to go to bed only when sleepy, be sure you actually are sleepy and not just tired.
The "seven deadly sins" formulated by the medieval monks included Sloth. The Bible in Proverbs 6:9 includes the line: "How long will you sleep, O sluggard? When will you arise out of your sleep?" But a more nuanced understanding of sloth sees it as a disinclination to labor or work. This isn't the same as the desire for healthy sleep. On the contrary, a person can't do work without rest periods and no one can operate at top performance without adequate sleep.
Can an animal be lazy? This question seems nonsense as laziness has a moral dimension and we do not think of animals as moral agents.
The Puritan work ethic can be adhered to and respect still paid to the sleep needs of healthy humans. It is wrong to see sleep as a shameful activity. On the contrary, we want to celebrate sleep as a good in itself.