The Bearded Dragon requires a body temperature of 93 – 110 degrees Fahrenheit to maintain good health. When it reaches this temperature, the dragon often opens its mouth. This activity is called basking and allows the Bearded Dragon to dissipate extra heat and enjoy basking.
While open mouth breathing can be a sign of respiratory distress in some lizards and dangerous in snakes, it is perfectly normally in Bearded Dragons and some other lizards.
Next time you see your Dragon sleeping open-mouthed, don’t panic, he’s just ridding himself of unnecessary heat.