My dog, Uku-Chan like to sleep in a position of belly up. It's may looks uncomfortable for us but it is the sign of Uku-Chan had fully trust on his environment (in my house) and try to cooling down his belly while sleeping. The other main reason due to our country hot weather.
My country where I stay, Sabah, Malaysia don't have 4 seasons but only had hot and rainy season for the whole year. Furthermore, my house very rare to switch on air-conditioner and depending on ceiling fan in order to save electricity bill. Uku-Chan might feel hot indoor, so I often saw the lying sleep posture when he sleep.
Uku-Chan too cute and adorable. Sometime, I will disturb him while he sleeping by rubbing his belly. Fortunately, he would not bite me instead continue sleeping or feel happy while rubbing his belly.