Fatigue is extreme tiredness that makes everything feel harder. When you experience fatigue, your body feels heavy, your mind feels foggy, and even simple tasks require extra effort. Unlike ordinary sleepiness that goes away after a good night's rest, fatigue can persist even after sleeping, and it can come from intense physical or mental effort or from illness. You might feel fatigue after running several miles, studying intensely for hours, or recovering from an illness.

Physical fatigue affects your muscles and body. A gymnast might feel muscle fatigue after practicing difficult routines all afternoon, making it harder to nail that final landing. Mental fatigue affects your thinking and concentration. After taking several tests in one day, you might feel mental fatigue that makes it tough to focus on your homework that evening.

Fatigue can also be a verb, meaning to make someone or something very tired, or to become very tired.

Fatigue can also describe materials wearing out from repeated stress. Engineers worry about metal fatigue in airplane wings or bridge cables: tiny cracks that form when metal bends back and forth thousands of times. Just as your muscles get tired from repeated use, metal can weaken from repeated stress.

Rest and recovery help overcome fatigue, whether you're an exhausted athlete, a worn-out student, or a bridge cable that needs replacing.