Moon-Mad

Question: How did the moon get so associated with madness and mental illness?

You know, we don't actually know the answer to that completely? The folk belief that lunacy (named after Luna, the Roman goddess of the moon, in our modern English terminology) is connected to the moon is a very ancient one, with mentions of it cropping up in ancient Babylonian texts and all the way through to the modern day. Nobody really started writing down why the moon should affect peoples' minds until comparatively late in mythological development, probably because before that it was one of those folk beliefs that was simply common knowledge, unquestioned and less than understood.

The earliest really firm explanation of lunacy is from Aristotle; he believed that the mind was the "wettest" or "moistest" part of the body, and that therefore the moon's phase would affect it the same way it affects the tides of the ocean and other bodies of water. Various cultures have always noted that there are obvious things affected by the moon, including peoples' bodies in the case of womens' menstruation, so it wasn't a stretch for them to believe that more things might be affected by it as well. Legends of things like lycanthropes, hungry moon deities who became ill or mad during the different phases of the moon, or magical creatures that could only be seen by the light of the full moon all contributed to a general idea that the full moon could and did cause strange things to happen, and many of those things could be considered forms of madness, which naturally opened the door to all madnesses being associated with the moon. Since there are some "madnesses" that only strike at night - for example, sleepwalking, nightmares or night terrors - some ancient peoples believed those must come from the moon, which was obviously the power over the nighttime world.

There are tons of scholarly theories about where people might have gotten this idea and whether or not it's actually true that people are mentally affected by the phase of the moon; even in the modern day there are a lot of very rational, scientific people who are utterly convinced that there is a greater chance for people to go fucking nuts on the full moon than at any other time, or who think everyone's moods automatically shift with the moon phase. Some have suggested that since menstrual cycles follow a lunar pattern and often include heavy mood swings or emotional changes thanks to hormonal shifting, the legend might have arisen as a result of women seeming to go a little more "mad" at certain times of the month, naturally ruled over by the moon; others have pointed out that since the moon sheds more light at certain times of the month than others, it might disrupt some peoples' sleep patterns more and more as it grows full, leading to them being irritable or having bad dreams that could be interpreted as madness; still others think it might be sociological in nature and lead back to an ancient survival instinct that tells people to become more alert and paranoid as the moon's phases change in preparation for the increased danger of the lightless nights that come with the new moon.

And, of course, there are also plenty of scientists who point to studies that show that there really isn't much correlation between moon phases and insanity at all, and who believe everyone who thinks there is is just chasing a bunch of red herrings in order to scientifically prove an old folkloric superstition.

Everyone has their own opinion on it, but most ancient peoples didn't explain why they felt the moon caused madness (except in cases where a god associated with the moon explicitly inflicts madness on someone, such as when Sin is said to cause insanity in those touched by specific rays of lunacy-causing moonlight when he's annoyed by someone); it was simply a fact of life that they all knew and believed, like the fact that plants need sunlight to grow. That folkloric belief is so strong that it's continued into the modern day, especially in western European countries and the Americas, still in that unformed, nebulous way that is not explained but only believed.

Having personally frightened the living daylights out of John with night terrors (seriously, I have straight up attacked him in the middle of the night without ever knowing it), I can definitely see how ancient people might believe that the moon, presiding cold and alien over the strange events of the night, might be the harbinger of madness and inflicter of insanity.