There is now a substantial body of data to suggest a relationship between sleep states, particularly paradoxical sleep (PS) or rapid eye movement (REM) sleep, and learning. In animal studies, PS increases have been observed following acquisition in formal tasks and following exposure to enriched environments (EE). Further, PS deprivation at certain post-training times is known to impair memory for recently acquired tasks. PS deprivation following EE exposure has been reported to impair the neural changes normally observed. It is argued that the prolonged PS increases observed following either formal training or EE exposure reflect the same kinds of neural change. It is proposed that accelerated neural plasticity takes place during elevated post-training PS and is especially vulnerable to disruption at specific post-training time periods called PS windows. Further, it is proposed that similar processes take place in the post-training sleep of humans.