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Name: Mackenzie
Status: student
Grade: 9-12
Location: PA
Country: USA
Date: Spring 2012

I was wondering if you could exactly tell me the relationship between body temperature and metabolism. When I sleep, why does my body temperature and heart rate go down? During the day, why does it go up?

Hi Mackenzie ,

The relationship is deeply rooted in the thermodynamics of biological energy demand. Being warm blooded means that the enzymes required for our general metabolism are most optimized at 37 degrees C(98.6 F). We tend to be most active during the day, when we stress our bodies(physically and mentally) and less active at night, when generally the body repairs.

Upon waking, our physically active period begins. Enzymes become activated and start processing any remaining food in order to meet that higher energy demand. The breakage of food chemical bonds releases heat. We begin to move, use more energy and seek more food. When we increase to our usual daily activities, even higher energy is demanded, we require more food, more water and more oxygen. Breathing, pulse, blood pressure and temperature increase. Muscle physiology causes further breaking of chemical bonds, creating more heat. For people enduring extreme physical exertion, the friction between tissues creates heat as well.

The heat cycle can not go on forever, we also must be cooled! Skin surface area for irradiative heat release, water for hydration, sweating, taking breaks and seeking a cooler environment helps to cool the body. 37 degrees needs to be maintained within a fairly narrow margin.

When we rest, the energy requirements decrease, breathing slows, heart rate slows, food passage slows. At sleep, the need for energy demand is dramatically decreased, the enzymes begin to deactivate, metabolism slows and food processing decreases to a very low level. Only repair, autonomic and maintenance mechanisms remain activated. Thus our temperature decreases and we generally seek a warm blanket. If we cool too much while asleep, the muscles will begin contracting... gently at first, to produce heat. The contractions can increase to the point they awaken the person and they start to shiver.

The models of the relationships are a bit too complex to address here. However, they re fascinating. Consider looking at:

Happy metabolizing!

Peter E. Hughes, Ph.D. Milford, New Hampshire

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