A good night's sleep in the outdoors can make the world of difference to the experience. We are often asked "How warm are our sleeping bags?" and this is an impossibly hard question to answer because there are so many factors that can influence a goods night sleep. S.L.E.E.P is an acronym we have developed to help you understand about these factors.
Surface- what are you going to sleep on?
It is easy to underestimate how much heat can be lost through the ground, in fact in some tests researchers have found heat loss through the ground to be 3 times that of heat lost to the air. A poor mat can make an expensive sleeping bag feel more or less useless wheras a good sleeping mat could have the effect of boosting your sleeping bag's rating. A good sleeping mat will keep you comfortable from the lumps and bumps of even the worse campsite whilst also providing a crucial barrier against heat loss. Any insulation in any sleeping bag will be compressed to almost nothing when you lie on it so the sleeping mat is crucial to stop heat conducting into the ground. Obviously the colder it gets the more heat you could end up loosing through the ground. An airbed maybe extremely comfortablebut the large uninsulated air chambers will increase the heat loss from beneath. A close cell mat commonly referred to as a "Karrimat" is extremely hard to compress making it an excellent insulator, a similar foam type is used to insulate pipes in lofts and unheated areas of the home. However a Karrimat isn't the softest of surfaces unless you sleep still on your back. Self inflating mats offer comfort and insulation. However they can be compressed especially at the hips and the shoulders which can result in cold spots. In cold conditions pairing a self inflating mat with a closed cell mat will help to prevent these cold spots whilst maintaining the comfort.
Layers - What are you going to wear inside your sleeping bag?
Layering clothing is second nature to most outdoor enthusiasts but most campers still use their sleeping bag like a bed at home. They crawl into it and then trust it is going to keep them warm for the next 8 hours or so. The colder it gets the more critical it becomes to ensure you reduce any heat loss from your body. Equally overheating in a sleeping bag will cause a build up of moisture inside the filling of the sleeping bag. To help regulate your body temperature in a broader range of temperatures think about using clothing and sleeping bag liners. You could even combine thinner sleeping bags together. Try never to sleep in the same clothes that you have worn during day. They will probably be damp and will cause you to cool faster. Heat loss from damp clothing can be upto 25 times faster than dry. Keeping a set of dry gloves, socks and thermals for sleeping in will boost your morale when it's time for bed. Wearing thermals will also lessen the blow when you have to crawl out of the tent for a call of nature. Thinking about what other items you may have with on your trip that could provide more insulation can help you save weight. If you have a down jacket or a fleece, using it like a mini blanket inside your sleeping bag will help insulated your core and may allow you to use a lighter weight bag. Ensure you use the hood and neck baffles to stop any "bellows action". Creating a good seal around the sleeping bag openings prevents your natural movements during sleep pumping warm air out of the bag and sucking in cold air.
Eat & Drink - Have you got enough fuel to keep you warm all night?
Think of food as a fire. To get it started you need quick burning kindling. These are your quick hit sugary foods like chocolate bars and carbonated drinks. You'll get a quick hit but it won't last very long. If you eat a Mars Bar for tea you'll probably feel warm as you climb into your bag but very soon you be feeling the cold and it is unlikely you'll want to get up and prepare anything extra. Complex Carbohydrates like pasta, rice dried fruit release their energy a little slower and like good dry sticks will keep your fire burning bright. These are fine if your keeping active and replenishing your fuel but to keep your fire buring all night you'll need some big logs on top of those sticks. These are the proteins like meats, fish, cheese & nuts and fats like margarine and those contained in your protein foods like cheese and processed meats. The body needs water to convert these food into energy. Failure to properly feed yourself or keep yourself hydrated when camping can result in a cold night sleep. Dehydration can be a big problem in the cold as it numbs our thirst mechanism making us feel less thirsty. This problem is further increased because your body needs to warm and humidify any cold air you breathe. As you exhale you can lose a significant amount of water.
Most people wake up between 3 and 4am feeling cold and lie there shivering until the morning. This is because you body didn't have enough fuel for the night or you didn't drink enough to keep those logs giving off energy.
Many of us like a little tippling especially around the campfire but alcohol dilates the blood vessels increases heat loss. This fine whilst we are sitting by the warm glow of a pub fire but will have a negative to our bodies when we go to bed in our sleeping bags.
Extremities - Can you sleep when you have cold feet?
Your head, hands and feet need to be kept warm to keep them working, this means they are packed full of blood vessels. Because they are packed full of blood they are also the first parts of the body to feel the cold. If your head, hands or feet are cold you will find it hard to get to sleep and it could have the effect of making the rest of your body feel cold. Wearing a hat in a sleeping bag and greatly effect how warm you will feel. Pulling on a nice warm dry pair of socks and wearing a pair of gloves will help keep your hands and feet toasty. Just like the layers of your sleeping system you add more or take them off depending on the conditions.
Protection - How are you going to stay dry?
What protection have you got to keep you and your sleeping bag dry and out of the wind? Where are you sleeping, a bothy, a tent, a snowhole or an Alpine hut. All of these will have an effect on your sleep. Staying dry is not just protecting yourself from precipitation but also dealing with your condensation is equally important. If your sleeping bag gets damp, can you dry it during the day. Can you minimise the draughts in the area you are sleeping, not doing so will increase your heat lose, especially when using thinner sleeping bags. Many people think using a bivi bag or sleeping bag cover like our Hunka will increase a bags warmth as well as keeping it dry. This isn't strictly true and it depends a lot on the temperatures you will be sleeping at. In warmer conditions sleeping inside a bivi bag can cause pools of moisture to collect, where as in colder conditions it will act like a vapour barrier system reducing the moisture intake into the sleeping system.