Here's what I would do, and I've done this lots of times, when I pull-up to the gym or my house (my gym) I just sit in my truck, lean my back (not the seat, thats a no,no) rest my elbow on the window edge with my palm on the side of my face and go to sleep for 5-10mins (it's so easy, because I pretty much want to
fall asleep at the red lights) it's hard to stay asleep in that position in the vechile for a long time, by the time I get out and start warming up I'm full of energy, I've done this at work also, take my last 10mins of lunch and put my head on the desk, lots of energy after that, powernaps man, beats taking supps.