He developed this

What is "this"? this ability?

Does he also get a full night's sleep? Eliminating other stages of sleep is almost certainly bad, but supplementing with REM seems to me unlikely to be bad.

People with narcolepsy basically only have REM sleep. Narcolepsy is very bad, but many people who eventually develop it seemed to have only had REM sleep when they were functional with no ill effects. In particular, they greatly benefit from naps (both before and after developing full-blown narcolepsy).