Here is an interesting reference: http://www.cdc.gov/mmwr/preview/mmwrhtml/mm4832a1.htm

They mention 878 to 1513 deaths per year due to CO poisoning (all venues, not just camping) and advise adequate clothing and bedding rather than a heater to stay warm in a shelter.

Probably more of a hazard than bears or lions, or even tiggers, but CO poisoning just doesn't grab the headlines....

You can sleep just fine with an open fire as long as you have a reflecting surface of some sort on the side opposite your blaze. Just be sure there is plenty of ventilation.

Sleeping cold isn't a whole lot of fun,but the good news is that you are sure to get an early start in the morning.