How do I stay warm at night in a tent?

Insulation from the ground rather than more blankets, eating before bed, and the ventilation that stops you waking up damp and cold.

Difficulty: beginner
Time: 30 min
Read: 4 min
Safety: warning

Short answer

Most of the cold comes from the ground, not the air. A sleeping mat with a decent R-value matters more than a warmer sleeping bag. Eat something before bed, wear a hat, and keep the tent ventilated so condensation does not soak your bedding.

People who are cold in a tent usually add more on top and stay cold, because the ground is conducting their heat away far faster than the air. Insulating downwards is the fix.

Safety

Never use a gas or petrol stove, a disposable barbecue or any fuel-burning heater inside a tent, porch or awning. They produce carbon monoxide, which has no smell and kills people in tents every year — a barbecue brought inside to warm up while it is still cooling is a recognised cause of deaths. Ventilate the tent even in cold weather. Hot water bottles must not be overfilled or boiling, and candles and any naked flame do not belong in a tent at all.

What you'll need

A sleeping mat with an R-value suited to the season
A sleeping bag rated for the conditions
A hat and dry sleeping socks
A hot water bottle (optional)

Step by step

Insulate from the ground first.The ground conducts heat away far faster than air. A mat's R-value measures this: around 2 is a summer mat, 4 or more is needed for cold nights. This is the single most effective change.
Double up mats if needed.A closed-cell foam mat underneath an inflatable one adds insulation cheaply and gives you a backup if the inflatable punctures.
Read sleeping bag ratings honestly.The 'comfort' rating is the useful number for most people; the 'limit' rating means survivable rather than pleasant. Many people, particularly if they sleep cold, need a bag rated well below the expected temperature.
Wear dry clothes to sleep in.Change out of everything you wore during the day, even if it feels dry — it holds sweat. A dedicated dry base layer and socks kept in the sleeping bag makes a large difference.
Do not wear too much.A sleeping bag works by trapping warm air around you. Wearing thick layers compresses the insulation and can leave you colder, not warmer.
Wear a hat.A large amount of heat is lost from an uncovered head, and it is the one part outside the bag. A thin beanie is the cheapest warmth available.
Eat before bed.Digestion generates heat and your body needs fuel to keep warm all night. A hot drink and something substantial an hour before sleeping is genuinely effective.
Warm up before getting in.A sleeping bag does not produce heat, it retains yours. Getting in cold means staying cold. A short walk or some star jumps first works.
Use a hot water bottle sensibly.A proper bottle, not boiling water, put in the bag twenty minutes before you get in. A well-sealed water bottle can work but a failure is a serious problem in a sleeping bag.
Ventilate the tent.Counter-intuitive but essential. A sealed tent fills with moisture from your breath, which condenses and drips onto your bag, and a damp bag is a cold bag.
Empty your bladder before sleeping.Your body spends energy keeping that fluid warm, and getting up at 3am is much colder than getting up at 11pm.
Pitch somewhere sensible.Cold air flows downhill and pools, so a valley floor or a hollow is noticeably colder than slightly higher ground. Avoid the very bottom of a dip.

Tips

The mat matters more than the sleeping bag. Cold comes up from the ground.
Never burn anything inside a tent. Carbon monoxide kills campers every year.
Ventilate even when it is cold — a damp sleeping bag is a cold sleeping bag.

Common mistakes

Adding more layers on top instead of underneath — Insulation under you is compressed by your body weight and does very little, so the ground keeps conducting heat away. Adding R-value below you is what actually works.
Sealing the tent up to keep warmth in — You exhale a surprising amount of water overnight. With no ventilation it condenses on the inner surfaces and soaks the sleeping bag, which then loses much of its insulation.

If it doesn't work

Cold from underneath

Cause: Mat R-value too low — Fix: Add a foam mat beneath the inflatable one, or use a higher R-value mat.

Wake up damp

Cause: Condensation from lack of ventilation — Fix: Open the vents and keep the inner door slightly open; use the porch for wet gear.

Cold despite a good bag

Cause: Got into the bag cold, or not eaten — Fix: Warm up first, eat something, and use a hot water bottle.

Feet always cold

Cause: Bag too tight at the foot or damp socks — Fix: Dry sleeping socks kept in the bag, and do not compress the bag's insulation at the foot.

Questions people ask

What R-value do I need?

Roughly 2 for summer, 3 to 4 for spring and autumn, and 5 or more for winter or frozen ground. Two mats add up approximately, which is a cheap way to extend a summer mat.

Can I use a small heater in the tent?

No. Any fuel-burning heater, stove or barbecue produces carbon monoxide, which is odourless and lethal in an enclosed space. Insulation, food and clothing are the only safe answers.