Brumation, also called hibernation, is the reversible state in which many Bearded dragons spend the colder winter months. It is characterized by a type of slumber or inactivity, where they are asleep most of the time. This behavior is also characterized by loss of appetite, hiding behavior, and sometimes excessive digging behavior.
During the brumation/hibernation period, unlike the true sense of the word, these animals will actually be awake some of the time. It is normal during this period to have some degree of weight loss and a depressed appetite, sometimes even total anorexia. This can last for weeks to months and usually start when the nighttime temperatures start to drop, even if the daytime temperatures are still warm and bright.
Not all Bearded dragons will go into, this so-called, slumber. Some juveniles can skip their first winter brumation. Some adults might also take this route and apart from being a winter thing, there is no set time or duration for this to happen.
In nature, the change in seasons will cause this behaviour to start and end. In captivity, however, nothing should be done to induce this behaviour. Lighting and heating should remain unchanged. The same goes for when these animals wake up, at which food should be introduced slowly again to gain normal body condition.