Getting outside is really important, because even on an overcast day there are loads of photons to be had. Unfortunately I don't think being out in the car counts as outside. Glass absorbs a lots of UV (which is why you don't sunburn through it) so there may well be some science in it. Perhaps what benefit you feel over when you were in windowless A&E is down to the placebo benefit...or just that seeing things, funny incidents in the street, smiles from passing someone you know, heartwarming sunsets etc are already a damn site better for you than being under strip lights all day. However, as I said, what you want is real outdoorsness, so you want to try getting out of the car more. Do calls come through to a bleep/phone or to the car itself? If its a bleep then you can stomp around outside in quiet spots. The fast pacing you'll need to keep warm will also kick off some endorphins which of course help with the mood.