The given temperature range tries to cover the optimal condition for a broad range of people. Optimal here means that below that many muscles that have to rest

The given temperature range tries to cover the optimal condition for a broad range of people. Optimal here means that below that many muscles that have to rest in the sleeping state are instead working to compensate the loss of heat (and with them also a bunch of neurons), above that you are kind of forced to move constantly to cool off by exposing the overheated parts of body. 15°C may be too cold for most, but it's ideal for overweight and obese people.