Having a good sleeping bag is important for getting a good night's sleep, as is a good sleeping pad. But having a sleeping bag and a sleeping pad that work together to keep you warm is better.
Sleeping Bag QuiltsSleeping bag quilts are a great example of this in action (Nunatak is a great sleeping bag quilt manufacturer). Sleeping bag quilts, for those not in the know, have an enclosed footbox, to keep your feet warm, and then open up on the bottom of the bag as it goes up your body. The justification for this design is that sleeping bags keep you warm through the loft created by the insulation. Any insulation under you is being wasted, because it is being crushed down. So why not remove it and save the weight?
Of course when you remove that extra material you are then sleeping right on top of the sleeping pad, which is where it helps to have a good sleeping pad.
An important metric, when looking at sleeping pads, is their R-value. This is a measurement of how warm the sleeping pad is. A sleeping pad with a high R-value will better reflect your body heat. The thing is, you actually lose a lot of heat out of the bottom of your body when you sleep. This is the reason that hammock sleepers tend to put something under them when the sleep. If they don't, all that body heat leaks out in to the open space below them.
Sleeping Bags for Restless SleepersSo a nice, warm sleeping pad is great when you have a quilt, but even with a more traditional sleeping bag, a good sleeping pad can really pay off. Side sleepers, for instance, or those who like to move around a lot, will benefit from a looser sleeping bag and a wider, usually rectangular shaped sleeping pad.
I personally am a side sleeper who likes to move around a lot, and my mummy-shaped sleeping pad is not enough sleeping pad to keep me off the ground at night.
Winter Sleeping BagsOf course the season that you are planning to sleep out in is important too. If you wanted to go winter camping, for instance, but did not have a sleeping bag that measured up to the winter temperatures that you were expecting, you could help to warm yourself up with a good sleeping pad.
Going back to the R-value discussion above, it would be easy enough to get a sleeping pad with a very high R-value. Exped, for instance, makes a really great one called the Downmat 9.
The high sleeping pad R-value will increase the warmth you get from your sleeping pad, and will supplement the heat from your sleeping bag.
So if you are thinking of picking up a new sleeping bag, sleeping pad or both in the near future, try and think about how you can get your sleeping bag and sleeping pad to work together for you.