Energy And Fatigue: Chronic Fatigue

Content provided by the Faculty of the Harvard Medical School

Energy and fatigue

The word "energy" can mean many things. You might use it to describe the strength you need to take on a physical challenge and the endurance to keep it up for an extended period: running a marathon, biking uphill, sightseeing all day, weeding and planting in the garden, and so on.

But energy is not just about muscles. It's also about the mind. When you're mentally energetic, you're alert, you're "on," you readily absorb information by reading and listening. Another aspect of mental energy is motivation — the drive to do things like read a book cover-to-cover in one sitting, initiate a new work project, or cram for an exam. You may have noticed that when you're really absorbed in an activity, you feel more energetic than when you are only half-interested in it. Motivation can be a powerful force in overcoming fatigue.

When you lack energy, you feel physically weak, mentally dull, or both. Effort of any sort can tire you out quickly. This absence of energy is often referred to as fatigue, and it's a common phenomenon that has been viewed differently over the years (see "Changing views of fatigue," below). Fatigue has physical, mental, and emotional components. Your muscles might ache. You might have trouble concentrating or need to read a passage over three times before you understand it. You might also feel unmotivated or bored.

Is it simply that you need more sleep? Sometimes when you lack energy you also feel sleepy. Sleepiness is specifically the urge to go to sleep. Most people need roughly eight hours of sleep a night. Some people can get by with less; some need more. Sleepiness can also be induced by medications that have a sedative effect on the brain (see "Medications").

But lack of energy is not just sleepiness: It's physical (weariness or weakness), emotional (lack of motivation or boredom), and mental (lack of concentration and sharpness). You can lack energy or feel fatigued without being sleepy, and you can also be full of energy and sleepy. Most people have had times when they've been able to override the urge to sleep with a surge of physical, mental, or emotional energy. But sleep and energy are related. No matter how energetic you are, you can't override sleepiness indefinitely. Eventually, lack of sleep will sap your energy and lead to fatigue (see "Sleep difficulties").

Last updated:	January 23, 2007