system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:Why does the heat make people feel more tired?
gpt
This is your body's attempt at regulating internal heat and avoiding hyperthermia (heat stroke).  Lethargic, slow, and confused behavior is an important sign of heat stroke, so be careful when you feel this way.

The more active you are, the more oxygen your cells burn and you generate more internal heat.  This heat MUST be radiated away from your body or you will die.
Normally it is not a problem, but when it is very hot the air cannot draw heat from you fast enough.  Also high humidity can make sweating useless.  Sweating cools you via evaporation, and if it is very humid your sweat cannot evaporate to cool you.  The mere act of producing sweat also requires yet more energy that adds to the fatigue.

By forcing you to "chill" out and relax, your body drastically cuts down on the amount of heat it produces, giving extra time for heat to radiate away.