According to the evolutionary perspective of sleep, the primary function of sleep is to:

The renewal effect

Explanation:

The renewal effect is the effect that the CR behavior gets returned when the same environment or context occurs after the extinction of the behavior. When the conditioned response gets extinguished the objects do not emit the same behavior for a long time.

But when the environment becomes to change the extinguished behavior renewed. Here the context is the stimuli that are presented by the organism in the environment. This effect gets stronger when the organism return to its original state.

Thus here Bart developed severe fear because of the phenomena called the renewal effect.