[AquaA]

My guess is maca is an energy booster so sort of has the same effect as adrenaline and noradrenaline (norepinephrine and epinephrine), so it would have a positive chronotropic effect on the heart by increasing the sympathetic stimulation which would cause increase in heart rate. And since the pontine center that is responsible for the REM sleep uses norepinephrine as a chemical transmitter it would explain why it causes the vivid nightmares since nightmares occur during REM sleep.
I didn't research this though, I'm just guessing since that's a quite an obvious link.
Would love to be able to have a study case about this. Get two people, give one maca and another a placebo, connect them to an EEG and just wait and watch till they get to the fast wave sleep. I'm sure the person on maca would have crazy beta and alpha waves.