Page 175 - Biobehavioral Resilence to Stress

152 Biobehavioral Resilience to Stress

by scientists at USARIEM indicate that dehydration effects on endurance

are less of a concern in the cold than in the heat (Cheuvront et al., 2005;
Pandolf & Young, 2000; Sawka, 1986).
It is generally well understood that human beings are better able to adapt
physiologically to tropical heat than to frigid cold. Numerous studies over the
past 50 years have documented the physiological changes that occur when

first exposed to hot-wet and hot-dry environments. These same studies have
further revealed how well the human body is able to sustain performance in
these environments without succumbing to heat injury (cf. Sawka, Wenger &
Pandolf, 1996). Individual responses to heat are varied and complex, but in

general, after repeated bouts of exercise in hot environments over the course
of 7–10 days, most individuals acclimatize in fairly predictable ways. Specifi -
cally, they show smaller increases in heart rate, more moderate and less rapid
increases in core temperature, improved peripheral (skin) blood fl ow, and

onset of sweating (less salty) at a lower core temperature. These changes have
a real and dramatic impact on physical performance capabilities. Pandolf and
Young (1992) found that most individuals who were unable to complete even

a moderate amount of exercise upon first exposure to a desert environment
could do so safely by their seventh consecutive day of exposure. Mountain,

Maughan, and Sawka (1996) identified the many aspects of how exercise
performance is improved: metabolic rate is lowered, cardiovascular strain
is reduced (lower heart rate, increased stroke volume, and better mainte-
nance of blood pressure), and fluid balance is improved (less electrolyte loss,

increased total body water and plasma volume). As for hypoxia and cold, the
warning signs of heat strain can be voluntarily neglected by highly motivated

individuals, often with disastrous consequences. Some well-known athletes
(e.g., Alberto Salazar) exemplify motivation to achieve performance beyond
safe thermoregulatory limits.

Sleep and Fatigue

Although the purpose of sleep is still not fully understood, it represents a
fundamental process that has been preserved across most animal species.
Current hypotheses concerning the need for sleep suggest that it may be
necessary to essential memory consolidation, restoration of brain energy
metabolism, and repair of oxidative stress in neurons. Deprivation of rapid-
eye movement (REM, or dream) sleep eventually causes death in a hyper-
metabolic state (Montagna, 2005). Whatever processes are involved, marked

differences have been observed in human susceptibility for sleep deprivation
and related performance degradation (see Figure 7.2).
Inadequate sleep can limit mental performance without aff ecting
physical performance. Severe sleep deprivation leads to progressive

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