Since they cool off after 15 minutes or so, they aren't much use for all day or night heat.
So, they are not all that great for all-night use...
Which leads me to...
hand-warmers powered by Zippo lighter fuel...
I know that they used to sell these things years ago. You can still get them used off of eBay, but not brand-new any more. Why? Did some people burst into flames using those things? If they are safe, I was considering grabbing one or two.