At a certain point, usually where one cannot whistle a tune or keep up a conversation because he is breathing too hard, the body basically stops burning fat and runs on straight glucose. Since most of us can't store more than 1200-1400 calores worth of glucose in the form glycogen, the body runs out of glycogen/glucose and must convert proteins into glucose. That, in turn, robs the body of the proteins/amino acids needed to repair, or rebuild the body.
So it is very important to keep the bodyt well supplied with those food items which can maintain one's glucose levels. Complex carbohyrates do this much more easily and better than proteins.
Yes, eating before hitting the sack, especially after a tough day in the mountains, is always best. Eating the proper foods throughout the day also helps alot. E
