I read a post on FB the other day, a page for beginners to wildcamping, asking for any tips on how to stay warm at night. So this is pretty much the same answer I gave there.
You’ll lose more heat to the ground, even in summer, than you do to the atmosphere. So the first thing you should do is make sure you can insulate yourself with a suitable sleeping mat. Once you’ve done that then try some of the following.
A decent sleeping bag liner can add an extra 5c plus if you need some extra insulation.
Wear thermal long johns and top, just make sure you wear synthetic materials, not cotton. Cotton loses it’s insulating properties when it gets damp, which it can do from sweating while you sleep (yes even in winter you can sweat). Top up with good socks, a fleece and hat if needed. Layers are your friend, the same as while you’re walking during the day.
Eat/drink something warm half an hour before you tuck yourself up, it can make a big difference.
Have a pee last thing, again it can make a big difference to how much heat you can retain.
Do a few exercises just before you get into your bag, the extra body heat generated will help keep you cosy. Do these just after peeing.
Always remember that the only heat source you have is body heat, therefore if you don’t generate it, and then retain it, you will not be warm.
There is no such thing as a warmer tent!!!
Tents do not keep you warm regardless of what some might say. It’s a fallacy, a myth. Less than 1mm thickness of material is not a good insulator, and therefore will NOT keep you warm. Tents do two things, they keep the rain off you, and they shelter you from wind, that’s it. Some will argue that a tent that reduces wind will help you stay warmer, that’s nonsense. All it means is that wind chill is reduced, but that’s not the same thing at all, do not be confused by this. If the wind stops and the temperature drops, will you remain just as warm? No, you won’t. I’ve known experienced campers fall foul of this particular fallacy because they believed their superior tent would save them and help keep them warm. It did not!
Basically it comes down to this, anything that can help insulate you or retain body heat will help keep you warm. The best way to do this is with layers. A mat is a layer, as is the part of the sleeping bag you lie on, the clothing you wear in your bag, and the liner you use inside it, and the part of your bag that is on top of you, they all form part of a layering system that will delay or reduce loss from the only heat source you have, body heat.
Hope this helps.