system
Given the following post, respond as if you're commenting on Reddit:
human
ELIA5: Why do I get sleepy when in a warm cosy car, but I need my bedroom to be like a fridge?
gpt
<- MD PhD Cand, Clinical Neuroscience, Neurosurgery Psychiatry spec.

We have a Circadian Biorhythm basically telling us when to sleep and when to be awake. This is directly connected to the hypothalamus, our "thermostat control center" for how hot our body temperature is and should be. 

Body temperature is altered by hormones like cortisol,adrenaline and thyroxin, and in turn muscle movements. 

While in the evening our body temperature is usually higher (Ever had a fever? These tend to go up at night due to this same process!), it has been shown that we experience a cooler room which lowers our temperature.*

This will slow muscle coordination and movement and thereby lower sensory input into our arousal system (e.g. the reticular formation) making falling asleep easier - it "relaxes" us.

In the afternoon a warm car on the other hand might stimulate this same effect by triggering associations of comfort. Psychoanalysis has an approach to understanding that this sensation of "comfort" will remind us of being protected by our mother. Biologically this effect of warmth + being "hugged" or "enclosed in tight blankets" stimulates the parasympathetic nervous system. This also calms our arousal system.

TL DR:
Cool in evening = relaxing muscles ->sends brain to sleep

Warm in afternoon = stimulates autonomous system -> sends brain to sleep.