I've found getting cold and getting comfortable enough to get
some sleep the biggest problems on a bivy. I've found bringing a
chemical hand warmer or two helps the morale when you are cold
because you have *something* that will warm you up.
Also, sleeping drugs help as well. My doctor prescribes some mild
sedatives that I take with along wee dram. Works great as long
as you aren't worried about hypothermia (depresses the metabolism).