Millets Basket Close Menu Search Store Finder Twitter Facebook Instagram Blog Engage voice search
Author avatar
Millets Author

The Science of Sleeping Bags

Who wants a cosy night’s sleep? Everyone, right? Even at home your temperature can affect how you sleep. We all have hot water bottles, an electric blanket or different togged duvets depending on the temperature to keep us warm in the cooler months. So, it’s easy to say that the sleeping bag is an important piece of kit to think about. There are many factors to consider when looking at the warmth rating, but we have to start somewhere…

Let's start with the outside temperature; the season you’re camping in is obviously a major factor. You may have a feeling for what temperatures to expect in the UK, but does that knowledge extend to include the night-time temperatures? In addition to seasonal effects, altitude also has a bearing. It might surprise you to hear that being just a few hundred meters above sea level significantly changes the air temperature.

The other variable to throw into the mix is how the cold affects YOU. Again there are many factors to consider but the important ones are your age, body mass and gender. As a rule, women, those who are slimmer, and the elderly feel the cold the most. Whereas, males, those who are larger, and those in their middle years feel it the least.

So where does this leave you? Pretty confused I imagine. Well fear not, help is at hand. To get you on track we’ve created a system to help you calculate a sleeping bag rating guide for UK camping, but before you get started you need to understand the sleeping bag rating scale.


Sleeping Bag Ratings

Sleeping bag warmth is rated in one of two different scales. Firstly, there are season ratings, and secondly EN13537 ratings. Season ratings (using a scale of 1 to 4) are how bags were traditionally categorised with 1 Season being suitable for UK summertime, through to 4 Season being suitable for year-round use. Although you will still see season ratings they have largely been replaced by the more reliable and versatile EN13537 ratings.   

For a sleeping bag to meet the EN13537 standard it must be independently tested to gauge its warmth. The test provides a range of temperature information for each bag, but the important figure for our purposes is the Lower Comfort temperature. This is the lowest temperature you can sleep in and still remain comfortable.

To help you gauge your Lower Comfort temperature for UK use take a look at the following table and select the correct figure for the time of year you would normally go camping. We’ve also included approximate season ratings in case the bag you are looking at is not EN13537 tested.


Time of year


Your Warmth Rating



Lower Comfort Temperature

Season Rating

High Summer only

(June to Sept)

10°C 1 Season


(May to Oct)

5°C 2 Season

Spring to Autumn

(March to November)

0°C 3 Season


(All year-round)

-5°C 4 Season


Now you have a starting point to work from to adjust your required rating to allow for other factors. The first thing you need to ask yourself is whether you generally feel the cold more. If, for example, you are a slim female who camps from May to October you’d probably want to adjust your rating from 5°C or 2 Season down to 0°C or 3 Season. Alternatively, if you are male and of a larger build you could probably stick at 5°C or 2 Season for the same usage.

The other major factor to consider is altitude. If your prefer campsites in the upland valleys of our national parks you may want to think about an extra degree or two of warmth. If, however, you enjoy wild camping closer to the mountain tops, a more drastic adjustment may be needed. For example, if you were an individual who feels the cold and enjoys upland wild camping from May to October you may find you need to adjust your rating from 5°C or 2 Season down to as little as -5°C or 4 Season.

Finally, for the most extreme camping this formula can be applied beyond our chart. If an average male were looking for a sleeping bag for year-round high level camping his rating would drop from -5°C or 4 Season to something closer to -10°C. 

Well we hope you feel better equipped to choose the right sleeping bag for your needs. Armed with this information and our huge range of sleeping bags we’re confident you’ll not get hot and bothered, or left out in the cold on your next camping trip.


Still confused? Leave your questions in the comments below and we will do our best to answer them.


Back to top
Author avatar
Millets Author

Related article

Which type of camper are you?

Which type of camper are you?

Whether you camp on your own or with the whole family, in the middle of nowhere or on a busy campsite, camping is one of the best past times for a healthy dose of fresh air. But which camper are you? Take our quiz to find out! Read more

Latest article

Wagging Tails & Walking Boots: PDSA Charity Dog Walk

Wagging Tails & Walking Boots: PDSA Charity Dog Walk

Recently Stuart, our resident outdoor adventurer, took a trip out to the Lakes with his Springer Spaniel Arlo for an epic fundraising walk organised by the PDSA, raising £1,500! We spoke to him post-walk. Read more


David 05-09-18 06:38
Hi. What is the best bag to take to Africa (Uganda, Kenya) in October November. Want the one with integrated mosquito hood. Love to hear your advice. Thanks
Quentin Brown 06-09-18 00:07
Whilst this is a good and helpful guide based on lower comfort temperatures I am also concerned with not getting too hot. Is there some information or further guidelines you can give in this regard. I hate waking up all sweaty or being unable to sleep because I'm too hot and after this summers heatwave with nights staying well above 20°C and tents turning into ovens the moment the sun is up choosing a sleeping bag that will not boil me in the bag is a significant concern.
Helen 13-11-18 20:15
Need one for lorry driver sleeping in his cab overnight. Want one which will wash in domestic machine.
Paul 17-03-19 11:37
Hi, I have a 3 year old boy and wanted to know if the Vango Junior Nite star is o.k for him? He’s just short of 100cm tall. We wish to use it between May & September. Thank you Paul

Join the discussion