There is more to it than temperature that is overlooked by brevity. When you're in a stressful, and uncomfortable, situation like that the numbness helps with the cold, possibly uneven, ground.

It can quiet the mind. And the inhibition can help nullify the fight or flight response aiding in getting to sleep.

There are several properties of alcohol that makes it seem like a reasonable deduction to drink a bit in order to sleep. Even if it is harmful and could compound the suffering down the line.