Before you go to sleep fill a sturdy plastic flask with hot water, to warm your sleeping bag. I have used Nalgene bottles for
this and never had a leak. DO NOT use metal flasks, such as made from aluminum or steel! Metal is an excellent heat conductor. If you fill boiling water into metal flasks the
metal will instantly become so hot you risk severe burns to your skin or melting the synthetic material of your sleeping bag or garment.
Nalgene bottles are made of a relatively thick plastic, which provides just enough insulation to keep them save for handling. Always be careful when handling boiling hot fluids though. I usually wrap the flask into a cloth, towel or a shirt - ideally made of cotton - before tucking it inside the sleeping bag. That way the warmth of the flask will last longer through the night, instead of giving off a lot of heat all at once and cool down fast.
A hot water bottle can also help with drying slightly damp clothes, like socks.
I have an insulation sleeve I use to keep the bottle warm. Mind you, I once filled a steel canteen with boiling water and stuck it inside such a sleeve. It melted the synthetic material of the sleeve!
Stick a bottle in the foot end of your sleeping bag, when you suffer from cold feet. Hug it to your belly to keep your body core
Pro tip: Instead of just filling hot water into your flask, use tea. A sweetened fruit tea is great. Or hot chocolate.
If you get thirsty during the night, or feel cold, drink a few sips of hot beverage. It will help you to keep warm.
You'll be gratefull to enjoy a sip of luke warm beverage first thing in the morning, before getting up and out into the cold, particulare when all other water has frozen over night.