Many have noted before that the pineal gland is also referred to as the "mind's eye"--a central metaphor in the show. The pineal gland secretes melatonin, a hormone known to affect our sensitivity to sunlight:
Only in very recent times have medical scientists identified the hormonal secretion of the pineal gland, finally isolating it in 1968. The essence was called melatonin, which means 'night worker' (from the Greek, melos, meaning 'black', and tosos, meaning 'labour'). Those with a high melatonin output react strongly against sunlight because it affects their mental capability; they are essentially night operatives. Melatonin is called the 'hormone of darkness' as it is produced only at night or in the dark.Also, those afflicted with overactive pineal glands tend to be very spiritually aware:
Exposure to an excess of sunlight actually makes the pineal gland smaller and lessens spiritual awareness, whereas darkness and high pineal activity enhance the keen intuitive knowledge of the subtle mind while reducing the stress factor.(Full text here.) What if those with the "sickness" have overactive pineal glands? I'm drawn to this speculation because it dove-tails neatly with my theory that the "sickness" is basically a cult of the island (or whatever controls it). It also connects well with the Lovecraft speculations. As anyone who has read Call of Cthulhu knows, the creature therein communicates with artists via vivid and disturbing dreams not unlike the mural. Lovecraft attributes these visions to the pineal gland.