I think those retreats you mention are Taoist retreats led by Mantak Chia. Their neurochemical hypothesis on what is occurring is certainly interesting, but as you point out speculation at the current point in time. (They propose that in prolonged darkness, the pineal gland secrets melatonin, and when levels extend beyond a certain threshold, pinoline, an endogenous MAOI is secreted, followed in turn by 5-MeO-DMT and DMT).
http://www.universal-tao.com/article/ancient.htmlNotice the profound similarities between these four experiences, despite being triggered by very different things.
"I became Consciousness facing the Absolute. It had the brightness of myriad suns, yet it was not on the same continuum with any light I knew from everyday life. It seemed to be pure consciousness, intelligence and creative energy transcending all polarities. It was infinite and finite, divine and demonic, terrifying and ecstatic, creative and destructive..." –
5-MeO-DMT account (Stan Grof)
"As I neared the warm glowing radiance ahead of me, I felt pure ecstasy. I was in the beginning of the light. I was part of the light…It was as if I had come home. I had come home to the beginning of not just me but the beginning of all eternity." –
NDE account (via Kenneth Ring)
"I was absorbed into this light, and this light became the entirety of space around me until I was only this giant, radiant light-filled void. I was real and home again and bigger than a trillion of our suns." –
Darkness retreat account (Lindsey Vona)
"The illumination grew brighter and brighter, the roaring louder, I experienced a rocking sensation and then felt myself slipping out of my body, entirely enveloped in a halo of light. It is impossible to describe the experience accurately. I felt the point of consciousness that was myself growing wider surrounded by waves of light. It grew wider and wider, spreading outward while the body, normally the immediate object of its perception, appeared to have receded into the distance until I became entirely unconscious of it. I was now all consciousness without any outline, without any idea of corporeal appendage, without any feeling or sensation coming from the senses, immersed in a sea of light simultaneously conscious and aware at every point, spread out, as it were, in all directions without any barrier or material obstruction. I was no longer myself, or to be more accurate, no longer as I knew myself to be, a small point of awareness confined to a body, but instead was a vast circle of consciousness in which the body was but a point, bathed in light and in a state of exultation and happiness impossible to describe." –
Account of Kundalini awakening (Gopi Krishna)
...all describe a sense of encountering a great light, a oneness with this light, a profound sense of ecstasy/bliss/love, a sense of infinity/eternity, and a sense of cosmic homecoming. Other very similar accounts crop up commonly among NDE and 5-MeO-DMT experiences, as well as OBE's, lucid dreams, psychedelics and spontaneous mystical experiences, and similar things have been described by meditators, yogi's and Taoists for a very long time. Intriguing how such different triggers can all seemingly lead to the same place.