Many people believe that exercise, like a hot cup of coffee, helps start the day with an alert mind and extra energy by warming up in the morning. Actually body temperature naturally varies only two or three degrees, dropping to a normal low during sleep and rising to the day's high during intense exercise. A warmup workout warms up cold hands and feet but does little to change core temperature.
Normal Temperature
People burn the least amount of energy while sleeping, when body temperature drops to about 97 degrees F. Normal activity quickly raises that to the average temperature of 98.6 degrees F. During light exercise, the energy expenditure stays low -- from about 210 Watts for housecleaning to 125 Watts for office work, according to D'Youville College. People conserve body heat by automatically restricting blood flow to the surface of the body. The first signs of real heating include warm hands and feet and a ruddy complexion.
Waste Heat
Exercise consumes fuel stored in the body as glucose or fat, but only part of that energy actually produces mechanical work. Only 25 percent of the energy that muscles produce moves objects and shifts weight. Often 80 percent of our energy burn results only in waste heat, says the University of Cincinnati. Sweating marks the first rise in core temperature caused by exercise, beginning when the internal temperature reaches 100.4 degrees F. Humans tolerate internal temperatures as high as 102 degrees F safely, but if exercise during hot weather brings core temperatures to 103 degrees F, the body slips into a potentially fatal condition called heat stroke. Even moderate activity in extreme conditions could cause heat stroke. Running at a moderate pace of 12 minutes per mile burns four times as much energy as light housework, and most of that power is waste body heat.
Circadian Rhythms
Changes in body temperature follow a regular daily pattern or circadian rhythm which links directly to feelings of energy or fatigue and plays a part in genuine depression. Slow starters who feel at their worst in the morning may benefit from a quick early workout simply because the exercise raises body temperature to normal faster than the ordinary routine. Exercise does nothing to change basic circadian patterns, according to a study published by the American Physiological Society.
Heat Stress
Although sensible exercise seldom causes dangerous overheating, doing your normal daily workout during exceptionally hot and humid weather could put you in the danger zone. Sweat evaporates more slowly in high humidity, losing some of its cooling effect. Determined athletes who push on beyond the many warning signs of heat stress risk serious and even deadly complications. Dehydration and loss of salts through sweating causes weakness, muscle cramps, fatigue and other unpleasant symptoms of heat exhaustion. Heat stroke victims stop sweating and become confused, losing consciousness as core temperature rises out of control. Anyone suffering heat stroke needs artificial cooling and immediate medical attention, according to the Centers for Disease Control and Prevention.
References
- CHE114 Web Site: Thermodynamics and the Human Body
- Raymond Walters College: Body Temperature Regulation
- Harvard School of Engineering and Applied Sciences: Thermoregulation
- Centers for Disease Control and Prevention: Extreme Heat -- A Prevention Guide to Promote Your Personal Health and Safety
- American Physiological Society: Intermittent Bright Light and Exercise to Entrain Human Circadian Rhythms to Night Work; Erin K. Baehr, et al; July 1999
