Where has your get-up-and-go gone?
In its broadest sense, fatigue is defined as a lack of energy, a subjective feeling of being tired. Fatigue also occurs when muscles decline in force — in other words, they poop out with overuse.
Most people complain of fatigue at one time or another, says Julie C. Haviland, a physician at St. Vincent Mercy Medical Center in Toledo, Ohio.
"To sleep? perchance to dream! Ay, there's the rub."
— William Shakespeare Hamlet, III:1 (1601)
It's entirely possible that Shakespeare wrote these words while fighting a bout of insomnia. But even if they were written during the day it's obvious that, more than 400 years ago, the Bard understood the importance of getting a good night's sleep.