While you are out camping, you will want to make sure you're able to keep warm at night. There are quite a few things you can do to keep warm on a cold night while inside a sleeping bag.
First, you will want to make sure that you are drinking plenty of fluids throughout the day. Keeping your body hydrated will increase your circulation, pumping blood out to your extremities and maintaining your body heat. However, you will want to slow down on hydration in the evening to reduce trips into the cold air to use the bathroom. Food is another important element in keeping yourself warm. Eating a big dinner before crawling into your sleeping bag can help you stay warm as you burn the calories you've taken in. Taking a small snack with you into your sleeping bag can help you refuel in the middle of the night. Remember, all food must be sealed in an airtight container so the threat of predators is reduced.
Selecting a good campsite can help you stay warm overnight. To find a good campsite, carefully survey the area where you will camp before pitching your tent. Find the highest point in the area that is also blocked by trees. The low-lying areas will hold cold air in, as heat rises. Trees can help keep the wind off of you during the night. When making camp, you will want to make sure that the area that you will be sleeping on is well insulated and up off the ground. You can use a layer of leaves or you can place a mat underneath your sleeping bag. Lay out your sleeping bag and ensure you have enough insulation under it, then fluff your sleeping bag before crawling in. It is beneficial to use a multilayer sleeping bag, as it will provide additional warmth and insulation.
When you get ready to sleep in your sleeping bag, there are several things you will want to wear. Have a clean, dry set of long johns to use as a sleeping suit, and wear a stocking cap. A lot of body heat can be lost through the head, and the cap will help keep you warm. Dry socks for sleeping can help keep you warm, and can help keep moisture out of your sleeping bag. You will also want to sleep with your nose and mouth outside of your sleeping bag to reduce the moisture retained within your sleeping bag. Moisture within a sleeping bag can lower the temperature inside your bag as it evaporates.