Be ready for the night

It's your first night on your long-awaited camping trip and you're dog tired. It took all the energy you could muster to pop open that can of beans, fire up your stove, and replenish your aching body with much-needed calories. Gratefully, you crawl into your tent and slide blissfully into your sleeping bag. Rest at last. Hiking over the big pass tomorrow doesn't seem quite so impossible. Until you wake up in the wee hours of the morning, teeth chattering ferociously, and the rest of your body feeling as frozen as a side of beef in a meat locker.
Getting cold in the middle of the night, or so hot and sweaty that you can't sleep, doesn't necessarily mean you didn't buy a good sleeping bag. More likely, you just bought the wrong sleeping bag. Since there are literally hundreds of models of sleeping bags available, perhaps even thousands, this happens more often than you would think.
While it is always important to consider how you plan to use equipment, sleeping bags tend to be more of a general-purpose general purchase. However, if you are planning on serious winter camping (in very cold weather) or mountaineering, you may well want to invest in a winter bag that is rated to twenty or thirty degrees below zero, in addition to a lighter bag for three-season use. Alternatively, some bags offer zip-out linings, so you have a double bag for cold weather, and your choice of the lining or outer bag for warmer conditions.
In general, three-season bags are rated to about 20 degrees above zero, which will work for warm conditions and usually be sufficient for brisk spring and fall nights. However, remember that there is no universal standard for bag rating. Also, people sleep at different temperatures, so while a 20-degree bag might keep your companion warm on a cold night, you might freeze in the same bag.