A warm tent is crucial mainly for a good night sleep and as a result having your mind and body function properly when you're back to hiking.
If you're like most hikers and don't pack a heater, you’re going to have to implement following scientifically proven methods to heat your tent.
1- Pour boiling water into your non-plastic bottle
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.
DO NOT sleep with anything hot around your torso. It can heat your torso too much, cause all the water in your organs to vaporize and can be very dangerous for your kidneys.
DO NOT drink the hot water inside any plastic bottle. Not even when the water gets cold.
2- Use hot stones
This one is a little pro tip but done correctly, 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:
1. A campfire
2. A few socks
3. 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.
Step 1. 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.
Step 2. 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.
Step 3. Pull the socks on over your hand and reach all the way down to the toes.
Step 4. Grab the rocks, turn the socks inside out and wrap them.
Step 5. 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.
Again, do not place them very close to your torso.
3- Reduce inner tent volume as much as possible
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.
How to stay warm in a tent
Although the purpose of this post is to give instructions about heating a tent without electricity, don't ignore the 2 additional tips below if you're mainly here for feeling warmer inside the tent.
1- Eat 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.
2- Stop drinking anything 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.
3- Keep your head warm
This one is self explanatory.
I've seen so much people focusing on keeping their body warm and ending up neglecting their head completely.
Your head is 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.