In summary, for me, heating a few stones before bed by the campfire and using them as a heat source inside your tent is the most effective (and ingenious!) method of all. Additionally; also consider preparing a hot water bottle, minimizing inner tent volume, and maintaining the warmth of your tent once it's heated.
Now let's take a closer look at these methods.
1- Using hot stones
This one is a little pro tip, but done correctly, it can be very effective.
The principle is the same with heating water - unless it takes longer for rocks to be heated. But more importantly, it takes them even longer to radiate all the contained heat until they die out. As a result, you're kept warm longer.
Things you'll need:
- A campfire
- A few socks
- A few fist sized rocks with smooth surfaces. Rough surfaces and sharp edges aren’t preferred because (1) they can cut off your socks as they'll be wrapping rocks and (2) when in contact with your legs, they would feel uncomfortable.
- Place rocks 5-10 inches away from campfire depending on fire's size and magnitude. Turn them around every now and then to make sure the center is being heated from all the angles.
- When rocks are just too hot to touch, you know the center is heated up to the necessary degree. It should take around 1.5 hours for them to be thoroughly heated.
- Pull the socks on over your hand and reach all the way down to the toes.
- Grab the rocks, turn the socks inside out and wrap them.
- Place them wherever you'd like: 4 of them at each corner or all at the center of your tent, around your sleeping bag, or, if you're comfortable with it, even inside your sleeping bag, just like boiling water.
2- Preparing a hot water bottle
This would work best if you're planning to sleep in a sleeping bag.
Before going to bed, the last thing you're doing should be boiling water to prevent it from getting colder.
Once boiled and stored in a non plastic bottle, put it lower in the sleeping bag around your feet. Heated air around your lower body will diffuse upwards and keep your entire body sufficiently warm.
It can heat your torso too much - causing all the water in your organs to vaporize. Can be very dangerous for your kidneys.
DO NOT drink the hot water inside any plastic bottle. Not even when the water gets cold.
3- Minimizing inner tent volume
Less air inside will be easier to heat.
Use your current tent in smallest volume - or buy the tent with the smallest size - you can get away with.
4- Maintaining the achieved warmth
Although the purpose of this post is to give instructions about heating a tent without electricity, do not ignore the 2 additional tips below if you're mainly here for feeling warmer inside your tent.
Eating before bed
Your body produces heat as side effect when it’s digesting food.
Since carbohydrates are digested faster, save proteins and fats for night time to ensure even longer heat production when you're asleep.
No drinking at least 4 hours before bed
This is the easiest tip to implement.
Let's say you heated inside your tent using the methods explained above. As you see, most of them aren't easy.
Unless you want all your hard work go down the drain, don't get out of your sleeping bag and tent at night to pee and let the warmth inside move out.
Obviously, this comes down to making sure you're not going to have to pee - so stop drinking anything at least 4 hours before sleeping.
Keeping your head warm
This one is self explanatory.
So many people focus on keeping their body warm and end up neglecting their head completely.
Your head is just as important as your body when it comes to feeling warm. Your body is already inside the sleeping bag, so never forget to wear a beanie while sleeping. If you don't have one, simply warp your head with whatever you have.
I'd prioritize the first trick (use of hot stones) above all else. It's my favorite one by far and works like charm. Simple too. Even simpler and easier to do than boiling water.
Also, maintaining the achieved warmth of the tent is crucial. I'd highly recommend paying a lot more attention to this too.