Even though this position looks slightly odd, it’s nothing to be concerned about — this just means that they’re very confident and secure with themselves, and with their home environment.
A dog that sleeps on his back is a happy, relaxed dog enjoying a deep sleep. They are letting it all hang out. If you see them doing this in warm weather it’s because they’re keeping cool by exposing their belly.
This makes sense especially since sleeping on the back translates into being the most vulnerable position for a dog. You’ll most likely never see wolves sleeping on their backs in the wild, as this sends a message of vulnerability and submission.
Dogs who assume this sleeping position usually tend to be very laid-back and have an independent streak.