and on that page it is written that REM sleep is lowering your serotonim level and that the lack of serotonim can cause depression so i wonder have you skilled lucid dreamers had any problems with that?
also it lowers your serotonine level, because your brain is active (the neurotransmitters it produces are used (depleted))
If your are lucid or not does not matter, because in rem sleep the brain is active no matter what.