Prominent among theories of the function of sleep is Meddis' immobilization hypothesis, which holds that sleep, rather than serving a restorative function, plays a protective role during times that animals cannot be usefully engaged in other activities. Meddis reasoned that animals not immediately threatened by predators would be safer if they passed the time sleeping. Sleep would prevent an animal from moving or responding to nonthreatening stimuli in ways that might attract the attention of predators. However, that hypothesis cannot easily explain why one often observes a rebound in sleep time or intensity following a period of sleep deprivation. Neither does the hypothesis explain the existence of various states of sleep, which themselves may be associated with different functions.