Choosing a sleeping bag can be quite complicated, in this video Ian talks you through comfort ratings and season ratings.
First Ian looks at one season sleeping bag, the lightest and most compact sleeping bags
Next he shows you two season sleeping bag, which are really good from late Spring to early Autumn. After that there is three season sleeping bags, used from early Spring to late Autumn and on milder winter nights. Also three season sleeping bags are brilliant for school expeditions and award schemes.
After that there is four season sleeping bags, which are used in winter, where it goes well down below freezing.
Finally the five season or expedition sleeping bags, which are incredibly warm and used in high mountain situations – where temperatures get down to -40
Personally, Ian likes to choose his sleeping bags based on comfort rating. Which will have an upper and lower temperature rating, and within those temperatures is where the average person is likely to be comfortable.