Sometimes during the workday I get tired, so during lunch I skip the food and take a nap. I’ve got an Element, so there’s lots of room to spread out. It’s great during the cooler weather - just open the little wing windows and it’s really comfy.
But - it’s warming up and it will get (literally) deadly hot in the car during the day. If I want to run the air, should I just turn the key enough to get that running, or should I go all the way and start the engine? One of my concerns is the battery - I drive about 5 miles to work, so will that be enough to keep it charged? And if I do run the engine, how much gas am I wasting.
(Maybe I should just curl up and sleep under my desk. 
)
If you start your car and sleep in it, you will really be cool; FOR GOOD! The carbonmonoxide will get you and the cops will chaulk it up as suicide!
Read your manual!
Outside? In a parking lot? Then why don’t I get carbon monoxide when I’m driving?
Engine not running = no air conditioning, car has to be running for it to work. And you’ll get poor a/c if the cars not moving, it would have to depend only on the radiator fan, rather than the air flow from moving. Any way to park in the shade? That’s the only way I’d want to try this in Florida.
It’s looking like napping in my office is the only safe thing. 
There aren’t big enough trees close to parking spaces, so no significant shade, alas. Or maybe I should just go to bed earlier…
Get a good CO detector, read the response time, and put it in the car when you take a nap.
In 1998, we moved from the Snow Zone to McAllen. I tossed our CO detector in the station wagon which was on a large trailer behind the rental truck. When we stopped at night, I heard a beeping. I found that enough CO had come back from the exhaust into the car that the CO detector was ringing.
Cars differ. I have found that some of them will produce enough cooling at idle to let you sleep.
However, you should take precautions against CO. The only way I know to do that is with a CO detector, but you need to find out if it really works. You might have to stay awake a few times to see if it works. I do know people can sit in a parking lot, wide awake, for a lengthy time, waiting for a spouse, and they do not die. I see them all the time.
When the car is moving you get much more fresh air flowing throught the car from the outside. However, feel free to try it.
Ding! Ding! Ding! You win the prize! Most folks could use an extra hour or two a night.
I used to do this all the time. Granted I live in Cleveland, but in the summer I would just roll down all the windows and and let the breeze come through. Even on days when it was close to 100? it worked fine for the half-hour I had for lunch. But I also worked in a welding shop that routinely was over 105? so it was like AC just walking outside.
Not only is it dangerous, but it is hard on the car and often not very effective.
Maybe I should just curl up and sleep under my desk Maybe a good idea. I am glad I had an office with a door before I retired. ?
Idle the car and run the AC. It will be weak at idle but better than nothing.
You can crack the window if worried about CO.
As far as wasting fuel you burn about 1/4 gallon/hr doing this.
Assuming it doesn’t have giant rust holes just above the muffler, I think he’ll be fine. People sleep in vehicles all the time - they’re called motor homes.
Try the AutoCool solar powered car fan.<<<<<<<
It fits above a slightly down window.
One issue no one has mentioned yet is that you won’t be awake to notice if your car overheats or one of the other warning lighs comes on. The risk is obviously small but not zero.