text: why is that when we sit in airconditioned place after exercising or sweating a bit, we feel sleepy? whenever i go work out, or sweat in the sun for some reason, why is that i feel sleepy/drowsy? is there any medical or logical reason for this. sources would be appreaciated. people naturally sleep better in a cold room or when the body becomes suddenly cold. this is due to part of your circadian rhythm. in addition to sleep naturally being effected by light (your internal clock, the superchiasmatic nucleus, is located near the optic chiasm where the optic nerves cross); most humans will wake easier by the light of the sun and sleep easier in the dark, there is also sleep regulation based on body temperature.   the human core body temperature changes slightly throughout a day. at around 3 am the body temperature is at its lowest. it starts to rise until near noon when it plateus and even dips a little bit (this results in people getting tired around lunch time...not because of the lunch itself, after all, you don't get tired after eating breakfast or dinner). many populations, such as mexico, take into account this normal fatigue and allow workers to nap (siesta) around that time. afternoon the body starts rising in temperature again until around 8 pm when it begins to fall again overnight.