I learned this the hard way when I was homeless at 17-18 years old in the middle of Wyoming winters. I had at least had my old Grand Cherokee to sleep in the trunk space, but not enough money for gas to run the heater. I'd pile all my clothes on top of me, but there were always patches where the cold got through and I'd spend so many nights just shaking from the cold. Spent the Christmas of 2004 this way. You are also correct that when it would be sunny (but still cold) the next day, it was an awesome experience. Almost like a ray of hope, as cliche as that sounds. It's crazy what having sunlight can do to improve your mood. That's one reason I loved living in SF later on, there were so many sunny days and I also wasn't homeless lol.