It will be interesting to see what kind of reactions you get from people on this site. When I was studying biology as part of nursing I remember joking to another student that if we asked to write about a gland in the exam, I was going to write about the pineal gland. The other student looked shocked. Of course, I was joking but as I was going to go into the psychiatrists branch of nursing, I was quite curious about the whole question of the role of the pineal gland, or the third eye.
I think that the link between the pineal gland and sleep is that it is involved in the production of Melatonin, which is a hormone involved in sleep patterns. This is connected to the ability to sleep in the day rather than night. Some people, especially night workers find that Melatonin enables sleep when their ability to sleep has been messed up by the disruption of having to sleep at unusual times. I managed to buy a bottle of Melatonin capsules and I found they helped me sleep a bit better, although not as much as the course of sleeping tablets my doctor once prescribed for me.
One of the main side effects of Melatonin is vivid dreams. That is interesting because it could be due to the role of the pineal gland. I have very vivid dreams generally, as well as borderline sleep imagery , especially when under stress. I wonder if this could be related to the role of the pineal gland too.
When you link the idea of the gland identified by Descartes to Buddhism, I think that the link is by means of the chakra system as conceived in Eastern philosophy. These are the main energy centres(or wheels), and other important ones include the heart chakra and, the root chakra as well as the crown chakra. The belief in the chakras is connected to a belief in a subtle body. The idea of a subtle body is important in the field of complementary medicine.
One important area of thought about the whole chakra system, especially the third eye is the belief in Kundalini. This has been written about by Gopi Krishna, who spoke of kundalini as an energy force, which runs from the base of the spine and genitals right up to the through the head to the third eye chakra. It can awaken spontaneously, through meditation practice or through the use of psychoactive drugs. Gopi Krishna speaks about the intensity of his own kundalini awakening , which he saw as an with 'madness', involving heightened but devastating experiences, involving sexual awakening and visionary experience. Somehow, he managed to balance his own kundalini experience.
The whole area of kundalini and the third eye is relevant for thinking about psychosis, although this is not the view adopted within mainstream psychiatry, which is focused more upon the role of the neurotransmitter, dopamine. The role of dopamine has been central to the whole development of antipsychotic medications.
However, the field of understanding the chemicals is one that is developing, and perhaps the area of the pineal gland (third eye) will be a missing link in understanding psychotic experiences. The most obvious aspect of this is the whole area of drug induced psychosis. Certainly, psychiatry has widespread recognition of the connection between the role cannabis smoking has in triggering psychotic episodes.
I am a bit tired, so don't really want to go into it fully but it is worth you looking up the ideas of David Icke in relation to reptiles. He is on the fringe, and went as far as suggesting that certain people, including the royal family in England shapeshift into reptilian forms. I am not saying that I agree with Icke's views and he has come under public attack, but I find his writings to be fun, simply because he has such an alternative perspective.