A big piece of SAD is related to Vitamin D production in the body, which doesn’t happen due to the eye at all, but due to skin exposure to sunlight. Vitamin D is converted from endogenous cholesterol (the cholesterol made in the human liver) by the radiation from sunlight. It’s sort of like the human form of photosynthesis.

Just a note: Normal peristalsis only goes one direction - increased peristalsis causes diarrhea. It does not cause the vomiting that is characteristic of these illnesses. Vomiting is very forceful use of muscles in the opposite direction than what they are used to being used for. Reverse peristalsis is the muscular mechanism for vomiting, but it is abrupt and very forceful.