"Day-camping" is what it is called
Perfectly legal to sleep in your vehicle during the daylight hours (in all but a few cities) wake at or before midnight and drive off to your next destination or simply run your errands (great time to go shopping, or visit your 24 hour gym for a hot shower, grab a bite to eat at your 24 hour diner, etc) and drive to your next destination as the sun is rising. Great time to be driving too; no traffic, beautiful scenery, and so on. Arrive, and go see the sights before the crowds show up, then find a nice park or parking lot to fall asleep around 2pm again. Rinse and repeat.
Great cold weather schedule as you are driving (or inside some building) over night and early morning when it is coldest outside, and sleeping when it is warmest. Much less reliance on a heater to stay comfortable.