During adolescence and early adulthood, our internal clocks think the day is longer than 24 hours. As a result, it's very difficult to fall asleep early or awaken early. During our late 20s and 30s, our internal clocks speed to about 24 hours and sleeping and waking go on schedule. Starting in middle age, our internal clocks may be shorter than 24 hours and waking up on time is easy. The internal clocks for the elderly speed further, which accounts for early bedtimes and early morning awakenings.
Caffeine is the most commonly used drug to self-medicate fatigue.
Researchers have confirmed that 200 mg of caffeine, equal to two or three cups of coffee, improves performance of monotonous tasks, such as long-distance driving. The improved performance continued during the third and fourth hours of the experiment. But caffeine is most effective when it's used infrequently, following two or three days of abstinence. Tolerance to caffeine quickly develops and larger doses are needed to achieve the same effect. Regular coffee use leads to physical dependence and a recognizable withdrawal syndrome, including headache, sleepiness, and lethargy.