Many dogs who sleep indoors opt for this "passed out" position. A dog will lie on its back in order to help themselves cool down as quickly and efficiently as possible. Most of the time, dogs who have expended lots of energy during the day, or who are overheated, will sleep flat on their backs. In addition, really happy, secure, and relaxed dogs will tilt their legs in the air during their sleep. Dogs who sleep in this position are highly confident in themselves, and they also tend to be very adaptable to different and new situations.