Csdrhrt

In Spanish we have the word lunático with the following meaning:

One who suffers from madness, not continuous, but at intervals.

This word comes from Latin lunatĭcus. According to Lewis & Short this word means both "living on the moon" (literally, I think) and "crazy person, lunatic". Did the Classical Latin word also convey the sense of "at intervals" in its second meaning? If so, then did the word derive from luna to reflect the fact that the madness was something that came and went as the moon phases, or was just to reflect that the crazy person appeared to live in a distant world? Did the word appear in Latin or did it come from an earlier language?

According to the Italian Wiktionary entry for the Italian word lunatico, lunaticus is actually a Late Latin expression and, in particular, a calque of the Greek σεληνιακός, seleniakos, and σεληνόβλητος, selenobletos, "epileptic, mad due to the influence of the moon", from σελήνη seléne "moon". This agrees with the description given by the Treccani vocabulary, which in particular states

dal lat. tardo lunatĭcus (der. di luna «luna») «che patisce di accessi di pazzia ricorrenti con le fasi lunari; epilettico»

that is, lunaticus was a person who suffered from brainstorms recurring with, or as, the phases of the moon.

Add to the highly interesting material above that in the ancient world the geocentric universe was divided between the supralunar (where order in the circular motion of the stars and planets reigned, in mirror image of the Prime Mover's) and the sublunar world (the Earth), where "disorderly", even chaotic, motions are the norm. Luna presides over that unruly world, as the rest of the planets over their respective regular circular paths.

According to this dictionary about the etymology of English words:

LUNATIC, lunatique, F. lunaticus, L. From Luna, the moon, because that kind of madness called lunacy increases with the moon. Luna qu. lucina, is from luceo, to shine.

Related, this other dictionary states:

lunatic: L. Luna, 'moon'; one too much under the influence of the moon, which, according to astrology, caused insanity.

So it seems inherent in the word that there is an intermittent pattern in the "disease".

Does this "lunar effect" have any scientific basis? Wikipedia states:

Two studies found evidence that those with mental disorders i.e. Schizophrenia generally exhibit 1.8% of increased violent or aggressive episodes during the full Moon, but a more recent study found no such correlation to that of nonschizophrenic human beings. An analysis of mental-health data found a significant effect of Moon phases, but only on schizophrenic patients. Such effects are not necessarily related directly to the appearance of the Moon.

An article in the Scientific American magazine provides an interesting analysis of this. It starts with some background:

ACROSS THE CENTURIES, many a person has uttered the phrase “There must be a full moon out there” in an attempt to explain weird happenings at night. Indeed, the Roman goddess of the moon bore a name that remains familiar to us today: Luna, prefix of the word “lunatic.” Greek philosopher Aristotle and Roman historian Pliny the Elder suggested that the brain was the “moistest” organ in the body and thereby most susceptible to the pernicious influences of the moon, which triggers the tides. Belief in the “lunar lunacy effect,” or “Transylvania effect,” as it is sometimes called, persisted in Europe through the Middle Ages, when humans were widely reputed to transmogrify into werewolves or vampires during a full moon.

Towards the end, it states:

According to Raison, the lunar lunacy effect may possess a small kernel of truth in that it may once have been genuine. Raison conjectures that before the advent of outdoor lighting in modern times, the bright light of the full moon deprived people who were living outside—including many who had severe mental disorders—of sleep. Because sleep deprivation often triggers erratic behavior in people with certain psychological conditions, such as bipolar disorder (formerly called manic depression), the full moon may have been linked to a heightened rate of bizarre behaviors in long-bygone eras. So the lunar lunacy effect is, in Raison and his colleagues’ terms, a “cultural fossil.”

I could not find the original references about this in Aristotle and Pliny the Elder.