OK. So I started with a steel water bottle and it didn't last longer than ~1am. So I switched to a Nalgene hoping the plastic wouldn't transfer the heat so quickly and last longer. I used it last night in mid 20s (*F) inside a thick wool sock and it lasted to sometime between 2-3am. I woke up chilled with a bottle of lukewarm water against me and had to kick it out.
I agree that the water bottle is awesome at warming things up and keeping you toasty warm but I'm stumped on how you guys are getting it to last through the night. I'm really hoping to replace the need for using the hand warmers on the cold nights completely but I'm not seeing it yet. Am I missing something here?