Confused by sleeping bag ratings? Not sure what type of sleeping bag you need? Here’s our guide to sleeping bag seasons:
In short: the higher the number, the colder your sleeping bag can go down to. The season ratings Alpkit uses for sleeping bag warmth are based on typical British temperatures in each season. Camping in the height of summer? You only need a one-season bag. Camping in winter? You’ll definitely need a four-season bag.
Here’s a breakdown of the season ratings:
1-2 season sleeping bags are designed for summer camping in typical British conditions (unless you’re hard as nails like Product Developer Rowan).
1-season sleeping bags are only really suitable for the height of summer. 2 season sleeping bags will extend your camping into late spring and early autumn – and give you a bit of extra reassurance on clear, cold nights.
Because they have the least insulation, these sleeping bags pack really small. This makes them ideal for ultralight summer backpacking and bikepacking, mountain marathons, or for use in warmer countries.
3 season sleeping bags are easily the most versatile and will keep most of us warm from Spring through to Autumn – the majority of the UK camping season. These sleeping bags often use ‘box wall baffles’, an internal divider between the baffles which prevents cold spots.
We all sleep differently, but with the addition or subtraction of a few clothing layers, 3 season sleeping bags can be used right down to the point where frost starts forming. If you only intend to buy one sleeping bag, a 3-season bag is the way to go.
A good balance of warmth, weight and pack-size makes three-season sleeping bags suitable for anything from wild camping beneath Munros, walking the Pennine Way or cycle touring through the Outer Hebrides.
4 season sleeping bags are designed for the very depths of British winter camping. These sleeping bags will almost always use a box wall construction to reduce cold spots.
If you’re a cold sleeper or are camping up in the hills, you may find you need to use a 4 season bag for spring and autumn as well. The outside temperature can reduce by between 1°C and 3°C for every 300m climbed, so you may need to go up a season from the valley temperature.
You would take a four-season bag on a winter climbing trip, a summit-top wild camp in the colder months, or even abroad, if you intend to camp at altitude. In short, any situation where getting your sleeping bag wrong could result in a very miserable night’s sleep… or worse… gulp.
If you’re buying a 5 season bag, you know you’re going to be in for some pretty extreme temperatures, either camping at very high altitudes or in the polar extremes of the planet.
These bags are highly specialised, sometimes using vertical baffles in the body for additional heat retention or more insulation in the hoods.
You would take a 5 season bag on a trekking trip to the Himalayas, high altitude alpine-style climbing or for expeditions to the Arctic circle.
There are many different factors, as well as external temperature, that determine how warm your bag should be. The key thing to remember is that you warm the bag. A sleeping bag is filled with insulation to retain heat, but it’s only as warm as the person in it. Consider the following factors:
Everything from your height, to your metabolism and your percentage of body fat can affect how warm you sleep. You probably already know if you’re a hot or cold sleeper based on what temperature you keep your bedroom and what kind of duvet you use. If you’re a cold sleeper, go up a season. If you sleep warm, you might get away with a lighter bag.
Alpine climbers have been known to eat whole wheels of cheese before bed to keep warm on their precarious high-altitude bivvies! When your stomach is full, your body converts the energy in the food into body heat – it’s basically like adding more fuel to your internal fire. But when the fuel runs out, the fire does too. If you’re using your sleeping bag after burning through thousands of calories, you might want to opt for a warmer bag.
You’ll probably have to ask someone else to find this out! Fidgety sleepers push warmth out of their sleeping bags, sucking cold air in. The alternative to a warmer bag is asking your mates to wrap you up in bandages like an Egyptian mummy.
After working out what the seasons mean and considering how you sleep, you now need to work out what you’re using your sleeping bag for and where.
Damp cold always feels colder than dry cold. Moist air and materials conduct heat better, making you cold quicker. That’s why a damp day in Scottish winter can often feel colder than a crisp day in the Alps.
The temperature drops dramatically with each 300m of height gain. Sleeping on the top bunk probably won't have much effect, but even one night at altitude can deplete your body’s energy stores – staying warm uses much more energy.
Conduction through the ground is the quickest way to lose heat whilst sleeping. If you are sleeping on frozen ground or snow, it is worth investing in a good sleeping mat as well as a sleeping bag. Deep mats with closed cell foam offer the best insulation.
A sealed up four-season mountain tent will keep you a lot warmer than an open tarp and bivvy bag, creating its own microclimate inside.
Whether down or synthetic, a damp sleeping bag is always colder than a dry sleeping bag. As well as losing a bit of loft, damp materials conduct your body heat away quicker.
Really, the most important factor is you. Yeah, YOU! You with a brain full of temperature ratings and seasons. Forget about sleeping bags for a minute: what do YOU like doing?
Are you a hardcore alpinist who thinks shivering on little mountain ledge in order to climb harder is worth the sacrifice? Or a mountain marathon enthusiast who’d cut your mat in half to shave seconds off your time? If so, you can probably tolerate a bit of discomfort for the weight and pack-size savings. Conversely, if you’re only going to use your sleeping bag for car camping, you definitely don’t want to compromise on comfort.
Seasons are a good starting point for working out what sleeping bag you need, but there are a huge range of other factors to consider. You will always be the best judge (yeah, you again!) of how warm a sleeping bag you need.
If you want a sleeping bag that’ll cover the biggest range of possible uses, we usually recommend a 3-season sleeping bag. You can always add a silk liner, some warm base layers, or a jacket in winter — and just sleep in your birthday suit in summer!