FATIGUE, BODY RHYTHMS, AND SLEEP ISSUES IN U.S. ARMY

Fatigue, Body Rhythms, and Sleep Issues in U.S. Army

Fatigue, Body Rhythms, and Sleep Issues in U.S. Army

Background Scientific Information

Two physiological factors that are known to have a significant effect on human performance and alertness are sleep and circadian rhythms. Some basic information on these factors is provided as a foundation for the subsequent discussion.

Sleep

Sleep is a vital physiological function, as critical for human survival as food, water, and oxygen, and as difficult to deprive as these basic physiological needs. Optimal sleep, both quantity and quality, is necessary for maximal performance and alertness. Reduced or degraded sleep can significantly decrease or impair performance and alertness. Sleep loss can degrade potentially any, and every, aspect of human performance, including memory, vigilance, decision-making, mood, and reaction time. doze loss can happen acutely and accumulate over time. Current scientific data have demonstrated that sleep of 2 hr less than an individual's required amount, is sufficient to significantly decrease subsequent performance and alertness. Therefore, an 8-hr sleeper who obtains only 6 hr sleep on one night can show a significant effect due to this acute sleep loss. Over time, doze loss can construct into a cumulative doze liability. Obtaining one hour less sleep than is typically required over 7 nights would result in a cumulative sleep debt of 7 hr, equivalent to about one night of sleep lost over a week. Recovery from sleep loss is accomplished through an increase in deep sleep and not through a one-hour for one-hour payback. Studies have demonstrated that the average adult human requires 8-8.25 hr of sleep for optimal performance and alertness. There is a range of individual sleep requirements around this amount (about 6-10 hr). Therefore, it is important to provide adequate sleep opportunities for an optimal amount of acute sleep, to minimize a cumulative sleep debt, and provide adequate recovery from a sleep loss situation.

Circadian rhythms

The circadian clock, located in the suprachiasmatic nucleus of the hypothalamus, regulates daily variations in numerous physiological processes such as sleep/wakefulness, temperature, and hormone release, as well as cognitive performance. An every day trough in body warmth happens about 1.5-2 hr before the common awaken time. This is close to the time of a daily trough in alertness and cognitive performance. Thus the circadian clock is programming the body for maximal sleepiness in the early morning hours (about 0300-0500 for a person on a typical sleep/wake schedule). There is a second period of increased sleepiness that occurs in the midafternoon, regardless of whether lunch was eaten. A human allowed living in an environment free of time cues (i.e., no external light/dark cycle or clocks) will extend the usual 24-hr day to a longer period, closer to about 25 hr. This extension is probably due, at least in part, to the natural internal tendency of the circadian clock to run with a cycle length that is slightly longer than 24 hr. This basic physiological property of the clock explains the relative ease of ...