1. When cats are deprived of REM sleep and then permitted uninterrupted sleep, the longer the period of deprivation—up to about 25 days—the greater the rebound of REM when they can sleep uninterrupted.

However, REM deprivation for more than 25 days produces no additional rebound. Speculate on a possible explanation. (Hint: Consider what happens to PGO waves during REM deprivation.)