Why is hot weather so exhausting?

Answer 1

Our body constantly tries to stay at a constant temperature that it works most effective at (38c/98f). When we get too warm our body needs to work to cool its self off by sweating and minimizing cellular respiration to keep heat generation down.

Decreasing the amount of cellular respiration going on means that there is less energy available for us to use which makes us tired, which conserves energy.

So we become exhausted for 3 reasons.
1). Our body starts sweating which uses up water to cool our selves down which when you become dehydrated you get tired.
2). We reduce the amount of cellular respiration which reduces the energy we have free to use which makes us tired.
3). Our brain actively makes us feel tired to encourage us to be less active so we generate less heat. (Our brain is tricky like that.)

Answer 2

Our bodies are designed to maintain an internal temperature of around 98.6F. Anything that challenges that temperature (hot or cold) requires an energy input in the form of sweating and other physiological responses in response to heat or constriction of blood vessles, faster metabolism or other physiological responses in response to cold. In both cases, fluid balance and electrolytes come into play. Especially in heat, the loss of fluids to sweating can deplete the body's water supply and change the electrolyte balance. Both of these can affect the autonomous brain to signal us to slow down, which it does by telling us we are fatigued. In case you are the one being exhausted by heat, make sure you drink plenty of water and make sure your electrolytes stay balanced. Little known fact: you can become dehydrated perhaps even more quickly in cold, dry air at high altitudes -- with the same effect of exhaustion.

Answer 3

Because activity generates heat, and the human body can only function well in a narrow temperature range. Females have a higher percentage of fat, and perspire less than males, many of whom also have the evolutionary advantage of thinning hair, which enabled their ancestors near the equator in Africa to hunt, scavenge and forage in the heat of the day, when others sought shade, being preferentially selected as mates (for being good providers - "bringing home the bacon") by the females.