Does anyone know why you get sleepy when you are inactive and your environment is hot? This seems to be more true if the surrounding air is stagnant.
yes. this always happens to me. especially when driving in Texas in the summer with no air conditioning. I hate driving drowsy so I usually pull over. I heard a recent statistic (on oprah so I don’t know about reliability/validity) that drowsy driving causes as many accidents as drunk driving. so DONT drive drowsy.
as for why this happens, I do not know. I like to get warm and cozy before I go to sleep so maybe my body just thinks it is time to sleep. 
tipi
When you move more, you produce more heat. This may be the reason you don’t want to move much.
Oh, I misread. I don’t know why you get sleepy but drink lots of water just in case. As a bonus you can’t doze off because you are constantly lifting the bottle or canteen and have to pee every ten minutes.
It doesn’t always make me sleepy, plus it often makes it impossible to sleep, its just too hot.
I would guess the body slows down from the heat and the blood has to go to the lower body to cool it, taking it from the brain.
I wanted to let this sit out there for a little while before I put my guess in. I originally thought it was due to sensory deprivation. As the temperature of the air surrounding you approaches your body temperature, the temperature sensing mechanisms in your skin would quit sending signals to your brain. I think the sensory deprivation theory is supported by the stagnant air making matters worse. If the air were stagnant, the hairs on your body would not be disturbed, thus decreasing the sensory input.
This started as a discussion among friends, and two other theories that I think are plausible are as follows:
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Oxygen Deprivation - As the temperature of the air increases, the air will expand, reducing the density of oxygen in the air, thus causing fewer oxygen molecules to be in any given volume of air. When you breathe in a lung full of air, you are inhaling less oxygen per volume of air, and possibly partially starving your brain of oxygen. If you’ve ever gone into a sauna, it does seem more difficult to breathe (but that may be due to your lungs not being used to the hot air).
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Defensive Mechanism - Your body tells you not to go out and make yourself hotter through physical activity by making you want to sleep.
Although I would like to be the one who is correct, so that I may gloat, I think the last theory is the one that is most likely to be right (if any of these are). What do you think?
What about people caught in snowstorms? They get mighty cold and then they sleep. So, can’t be the heat.
Probably cause you aren’t doing much cause of the heat or cold so you get sleepy?
What actually causes hypoxia at low air pressure is the partial pressure of the oxygen falls below the level required to transfer to the blood via the lungs. That is why you can breath as fast as possible in an altitude chamber and still pass out, even though there is plenty of oxygen around your body just can’t get at it. The really weird part is that you have no idea that what you are breathing is not doing the job, that is until you fall over 
Oblio