There are several ways to increase warmth when using sleeping bags:
It is very important to have a good quality mat. It is very important for people who are camping. If the cold is from the underground to the back, the cold is unbearable. If there are conditions, drink a hot drink before going to bed. Can make the body warm;
Eat a comfortable meal before going to bed, especially to add carbohydrates (starch, sugar);
Wearing a long thermal underwear and clean socks will be very effective;
Wear more clothes when the sleeping bag is not warm enough, or cover clothes and other items on the sleeping bag;
And more people crowded with a tent;
In the case of security, point the steam lamp or stove in the tent;
There is a fire. With so many methods, try it when it's critical.