Warm Sleeping Bags
In your case, it could have been you were just worn out from a long day and couldnt generate enough body heat to keep you warm. It could have been the bags faultmaybe it was an old bag, with down that had lost its loft or synthetic fill that was old or had some cold spots. You also mention you wore sweatpants and a sweatshirt with hood. Were they cotton? If so, bad choice. If they got even a little bit damp, they could chill you rather than keep you warm. And please dont take this the wrong way, but make sure the bag is rated to zero Fahrenheit, not zero Celsius. I once climbed Rainier with a group that had a member who made that mistake, and she damn near froze to death.
In any event, temperature ratings are imprecise and represent something of an ideal state. All kinds of things can throw them out of whack, from ambient humidity to ground temperature to your physiology. If youre tired, it always pays to eat a candy bar or something for a quick calorie jolt before bed. Make sure you have an insulating pad. Wear dry long underwear, wool or synthetic, including a hat. And socks. Finally, maybe you simply need a warmer bag. Every body is different in that regard.
Pick up a copy of the 2006 Outside Buyers Guide, on newsstands now, for a look at the warmest sleeping bags and 396 other torture-tested products.