Why you sleep cold, and the fixes ranked by what they actually buy

Most people who sleep cold blame the sleeping bag, and most of them are wrong. Heat leaves you by four routes, and the bag only addresses two of them. In practice the majority of cold nights trace back to the pad, to under-fuelling, or to fill that has quietly gone damp. The best three fixes cost nothing at all.

Where the heat actually goes#

Four routes, and it is worth knowing which one you are fighting.

Conduction to the ground. Your body compresses the insulation underneath you to nothing, so downward heat loss is governed almost entirely by the pad. On a thin pad this can be the single largest loss in the system.

Convection through drafts. Air moving through the hood opening, a zip that has crept open, or the gap at your shoulders carries warmed air out and pulls cold air in. Wind on a shelter also increases convective loss from the fabric, which lowers the temperature inside the tent, not just outside it.

Radiation. Infrared loss from exposed skin to the cold surfaces around you. Small in a closed bag, significant on a face outside it.

Evaporation. Moisture leaving your skin and your damp clothing takes energy with it. This is why hiking clothes worn to bed are a heat leak rather than a layer: every gram of water evaporating out of that fabric is heat taken from you.

A quick diagnosis: a cold back with a warm top half is conduction, a felt draught is convection, a cold face is radiation, and clamminess is evaporation.

The pad problem hiding inside most complaints#

The most common report we see is that a bag is "not warm enough", and the most common cause is the surface underneath it. Two reasons.

First, ISO 23537 tests bags on a specified mattress, so the comfort rating already assumes a pad of a given thermal resistance and a colder one silently invalidates the label. Second, conduction is unforgiving at low R-values: heat flow scales with 1 over R, so a summer mat at R-2 loses roughly twice as much downward as an R-4 pad in identical conditions, and four times as much as an R-8. The arithmetic is in the R-value guide.

The tell is simple. If the inside of your bag feels warm but your back, hips and shoulders feel cold, no bag on earth will fix it. Buy R-value or add a foam mat under what you have.

Fuel, water and the overnight furnace#

You are the heater. Everything else in the system only slows the losses, so anything that reduces your output shows up as cold.

Start from the physiology. ASHRAE defines one met as 58.2 watts per square metre of body surface, and an adult has roughly 1.8 square metres of it. Sleeping sits near 0.7 met, so 0.7 x 58.2 x 1.8 gives about 73 watts. That is your entire budget for the night, and it comes from food.

Going to bed under-fuelled after a long day means running a 73 watt heater on empty. A substantial evening meal and a slow-digesting snack keep the output up through the small hours, when core temperature naturally bottoms out. Dehydration works the same way: reduced blood volume means less efficient circulation to the extremities, which is why cold hands and feet often mean you drank too little rather than that your bag failed. Stove fuel planning covers the cooking side.

The hot water bottle deserves honest arithmetic rather than enthusiasm. One litre of water at 60 degrees C, cooling to the roughly 20 degrees C inside a bag, releases:

1 kg x 4.186 kJ per kg per kelvin x 40 K = 167 kJ

Your own body at 73 watts produces 73 x 3600 = 263 kilojoules in an hour. So the bottle is worth about 38 minutes of your own heat output, delivered fastest at the start and tailing off. It is genuinely useful for getting to sleep warm, and it is not a heating system. If you already carry a bottle that tolerates boiling water, it costs no extra weight at all.

Damp, and why night four is colder than night one#

A sleeping adult gives off water continuously, and some of it ends up in the insulation. Over consecutive nights without a chance to dry, loft falls. Down loses more than synthetic, as set out in the down against synthetic comparison, and the loss is cumulative: night four is colder than night one in the same bag at the same temperature.

Nobody notices it happening, because the change per night is small and the weather is a convenient explanation. The countermeasures are dull and effective: air the bag for an hour at any midday stop with sun or breeze, keep it off the tent walls where condensation collects, and vent the shelter properly. Managing shelter moisture is covered in tent condensation.

Every fix, ranked by degrees per gram#

The temperature gains below are a model, not measurements. They combine the ISO 23537 finding that the standard man and standard woman differ by about 6 degrees, the roughly 2 degrees C per full R-value rule of thumb for pads, and typical claimed values for liners and clothing. Treat them as an ordering, not a promise: the ranking is far more reliable than any individual figure.

Read the top of that table carefully. The first six rows weigh nothing and cost nothing, and between them they cover a range wider than the gap between a three season bag and a winter bag. Site selection alone can beat a bag upgrade: cold air drains downhill on clear, calm nights and settles in valley bottoms and hollows, so a pitch fifteen vertical metres higher, or on a slight slope where the air keeps moving, can be several degrees warmer for no effort. It is covered properly in pitching and site selection.

The bottom of the table is instructive too. A warmer bag delivers the largest single gain, but it is the heaviest and most expensive route to it, and it does nothing about conduction, drafts, damp or fuel. Buying it before exhausting the free options is how people end up with an expensive bag and the same cold night. Our sleep system calculator adds these adjustments up against a bag rating.

Things that feel warm and are not#

Alcohol. It causes peripheral vasodilation, sending warm blood to the skin, which feels like warmth and is in fact accelerated heat loss from the core. It also impairs shivering and judgement. Cold stress guidance from occupational health agencies is consistent on this.

A bag much larger than you. Extra dead air inside the bag is air your body has to heat, and a bag well over your length costs around 2 degrees against one that fits. This is also the argument for hood and draft collar cinches, which reduce the volume you are heating.

Sleeping in your hiking clothes. They are damper than they feel, and evaporating that water costs heat all night. Dry sleep clothes, kept in the pack and worn only in the bag, are free warmth in the strict sense: you were carrying them anyway. The layering system explains the same principle when moving.

Curling up tightly. It reduces surface area, which helps, but it also compresses the insulation at your knees and back into nothing.

A quilt in a draughty pitch. Quilts save weight by removing the crushed underside, but they depend on a good pad and a secure attachment system, and in a draught the penalty is larger than for a mummy bag. See quilts against sleeping bags.

Check this yourself#

You can find your own weak point in two nights without buying anything.

Night one, baseline. Sleep as you normally would and record four things in the morning: the overnight low from a thermometer at the campsite rather than the forecast, whether you woke cold, roughly what hour, and which body parts were cold. That last item is the diagnosis. A cold back means conduction. A cold shoulder or neck means a draught. Cold feet with a warm torso often mean fuel or hydration.

Night two, one variable. Change exactly one thing, ideally the free one your diagnosis points at: a different pitch, a foam mat under the pad, dry sleep clothes, or a proper meal. Do not change two. Record the same four items.

Two nights of this beats any amount of specification reading, because it identifies which of the four heat routes is costing you. The temperature calculator then turns that finding into a target rating rather than a guess.

Common mistakes#

Buying a warmer bag first. It is the most expensive fix and addresses only part of the problem. Work down the free list first.

Using the limit rating as a buying number. The limit figure describes a curled-up sleeper on the edge of shivering, about 6 degrees below the comfort rating. Comfort is the number to use, as explained in sleeping bag temperature ratings.

Trusting the forecast low. Forecasts describe a regional air mass at a standard height, not a hollow at the bottom of a valley on a clear night.

Overdressing inside the bag. Layers thick enough to compress the bag's own insulation reduce loft and gain nothing. Add a layer only while the bag still lofts freely.

Breathing into the bag. It feels warmer and deposits a surprising amount of water into the fill, which you pay for on later nights.

Ignoring the hood. An uncinched hood is an open chimney. Closing it to a small breathing hole is a free degree or two.

Frequently asked questions#

Why am I cold in a sleeping bag rated for the temperature?#

Usually the pad. Bag ratings under ISO 23537 are measured on a specified mattress, so a thin pad quietly invalidates the label. After that, the common causes are under-fuelling, dehydration, damp fill from consecutive nights, drafts through the hood or zip, and a campsite in a hollow where cold air pools several degrees below the forecast.

Does a hot water bottle actually help in a sleeping bag?#

Somewhat, and briefly. A litre at 60 degrees C releases about 167 kilojoules as it cools to bag temperature, which is roughly 38 minutes of an adult's own resting heat output. It is excellent for falling asleep warm and for warming cold feet, and it will not carry you through a night. Use a bottle rated for hot liquid and check the seal.

Do I sleep colder than other people?#

Very likely, if you think you do. ISO 23537 itself models a gap of about 6 degrees C between its standard man and standard woman, driven largely by differences in metabolic heat output per unit of skin area. Body composition, age, fitness and recent food all shift it further. A personal offset of 3 degrees from the label is common and worth building into your planning.

Should I wear my clothes to bed when camping?#

Wear dry clothes, never the ones you hiked in. Damp fabric next to skin costs heat continuously through evaporation, and hiking layers hold more moisture than they feel like they do. A dedicated set of dry base layers and socks that live in the pack is one of the cheapest warmth gains available, since you were carrying them regardless.

Does drinking alcohol keep you warm at night?#

No, it does the opposite. Alcohol causes peripheral vasodilation, moving warm blood to the skin where it is lost faster. The sensation of warmth is real and the net heat balance is worse. It also blunts shivering, which is the body's main involuntary heat generator, and impairs judgement about how cold you are getting.

Why does the same bag feel colder later in a trip?#

Because insulation picks up moisture from your body and the air, and loses loft as it does. The effect is cumulative over consecutive nights without drying, and it is more pronounced in down than in synthetic. Air the bag for an hour whenever there is sun or breeze, keep it clear of condensation on tent walls, and vent the shelter to reduce the humidity inside it.

Where should I pitch a tent to stay warmer?#

Not at the lowest point. On clear, calm nights cold air drains downhill and pools in valley bottoms, hollows and behind obstructions, so those spots can run several degrees below the surrounding slopes. Pick a slight rise or a gentle slope where air keeps moving, use natural shelter to break the wind, and avoid open ground under a clear sky where radiative loss is highest.