Beardies don't hibernate, but they will sometimes brumate. It's similar to hibernation but not a true hibernation. Their bodies slow down and they spend much of their time just sleeping or lying around doing nothing.
Typically, only older dragons will brumate.
You want to make sure that if your dragon is old enough to brumate, that the slow down is from brumation and not from an issue in your husbandry or your dragons daily diet.
-Brandon