Fear conditioning is an important survival mechanism, as is the ability to generalize learned fear responses to stimuli that are similar to the original conditioned stimulus. Overgeneralization of fear learning, prominent in many anxiety disorders, is however highly maladaptive. Because sleep is involved in the consolidation of fear learning, and in active processing of information, the present study explored the effect of sleep on generalization of fear learning. Participants watched a random sequence of pictures of a small and a big circle, one of them coupled with an aversive sound. Then, after a delay period containing either a nap or wake, generalization was examined as participants watched the two circles again, together with eight novel circles that gradually varied in size between the former two. Results showed that the fear response increased as a function of similarity to the conditioned response. However, there was no difference in the degree of generalization between the sleep and the wake group.