Fluctuation of sustained attention is an important aspect of synthetic speech perception because it is related to problems of vigilance which are crucial for real applications. To study whether and how sustained attention fluctuates in synthetic speech, we run an experiment where, during a long passage produced by a synthesizer or by a human speaker, subjects had to recognize computer generated clicks distributed randomly in the passage. The results showed differences in the fluctuation of sustained attention during the listening of passages produced by a TTS system with respect to the same passage read by a human speaker, suggesting that listeners of synthetic passages may be required to pay more attention than listeners of natural passages.