Sleeping Bag Season Ratings
These days the standard ways to measure a sleeping bag is in terms of a 'comfort' rating. This is an area where different suppliers can be creative in what they define as a 'comfort' rating. Their version of comfort seems somewhat different to mine. Typically a supplier will give an upper and lower comfort rating for the bag.
The upper rating will be the temperature at which the bag will feel too hot to sleep. The lower temperature rating in the comfort zone will be the one at which it is just about possible to sleep assuming that you are fully clothed and that you are in the foetal position.
One major factor that must be taken into consideration is that some people sleep 'hot' and some people sleep 'cold'. If your metabolism drops sharply overnight then you will need a warmer bag than someone who has a faster metabolism.
The second major factor is simply what you are used to at home or outdoors. If your central heating is always at 25°C regardless of the time of year and the weather then when you are outdoors you are always going to feel cold when compared with someone who lives in a much colder house.
These comfort ratings also assume that you are well insulated from the ground and that you are dry and that you are in still air. Any of these will have a huge impact on the sleeping bag performance. (See enhancing your sleeping bag performance).
After several years of use and packing and unpacking the sleeping bag will drop in its effective rating. My 3-year old 4-season bag is only just as warm as my new 3-season bag when used in the real world...
1 Season / +7°C
This is for 'summer' use only. The last bag that I bought - A Snugpak 650 - that had this rating was comfortable for me in temperatures between 14°C (fully clothed in a bivi bag) and 25°C (very little clothing, in a tent).
2 Season / 0°C
If you are only doing summer camping in the UK then I would suggest that this is actually the season rating to go for. It will be warm enough in most summer temperatures without you needing to worry too much. It would be an ideal 'festival' sleeping bag.
If you are out on the hills in summer and are using a bivi bag then this would probably be a good choice for a warm sleeper
3 Season / -5°C
A 3 season bag is going to be suitable for cooler weather. In practice for me I will happily use a 3 season bag in temperatures down to +10°C. In temperatures lower than this then I need to consider boosting my sleeping bag rating.
A 3 season bag is going to be too hot for summer use.
4 Season / -10°C
A 4 season bag will be good for autumn use and when the ground temperature is above 0°C I have not yet found a '4 Season' bag that I am comfortable in in sub-zero temperatures.
5 Season / Expedition -15 or more °C
In this category you are now most definitely hardcore. These are sleeping bags designed for people camping in snow and in arctic conditions. It is assumed that you will ONLY be using these bags when also fully clothed - which will include things like thermal trousers and puffa jackets and so on.
If you are not fully clothed then a 5 season bag will make a good bag for UK winter conditions in all but the most extreme of cases.
It is roughly accurate to say that the higher the season rating on the bag then the heavier and bulkier the bag will be. If you can only afford one bag then I would suggest that something like a 3-season bag is the most flexible. In the summer you can just wear underwear in it and in the cooler weather you can wear extra clothing in it.
If you are a little richer and do a lot of camping then it may be worth considering buying a 1-season, 3-season and 5-season bag. These 3 would then cover any conditions that you are likely to meet.
The best time to buy a sleeping bag tends to be around August/September when the sales are on and new stock is about to arrive. Big discounts will be available.
If you own both a 1 and a 3 season bag then using both together will probably be OK for winter but not extreme use - at the cost of a little extra bulk.