First, your metabolic rate is nominally three times higher doing camp chores and ten times higher hiking. As a result, the insulation you require is 1/3 and 1/10 of what you require to sleep at the same temperature.
Second, your heat loss when sleeping is directly proportional to the surface of your sleeping system. Arms and feet, as insulated appendages, significantly increase the amount of insulation you require to stay warm at the low temp rating.
By contrast, the BPL recommended concept of augmenting your normal camp wear insulation with a bag or quilt is a sound concept.