What is zen sickness?

I'm grateful for this thread. There are many great resources in the links.

The Dark Night of the soul article is particularly resonant. When I teach students meditation, I explain that these practices should come with a warning label, much like on every pack of cigarettes. There are real dangers in entering into this path of serious practice.

I found solace and many helpful tips on not destroying your life after awakening experiences in Daniel Ingram's book: "Mastering the Core Teachings of the Buddha" which he says he wrote so that others don't mess up their lives after these experiences as he did. https://www.mctb.org/

Kenshos happen. And like Hakuin's first one, they can distract from further progress and lead to narcissistic inflation of self-accomplishment and spiritual materialism.

Find a good teacher you can stay in contact with during the post-retreat integration and subsequent return to the rest of your life.

I still get occurrences of "Zen Sickness" after waves of intense wakeful experiences. No big deal. Just keep bringing it all back to chopping wood and carrying water. IMHO, Dilgo Khyentse's instructions on the "Every Day Practice" are perhaps the most cogent, compact pedagogy for the whole path. See this article on my website: The Everyday Practice by Dilgo Khyentse Rinpoche.

Zen Sickness, by Zen Master Hakuin mentions one cause of it was access striving. In Buddhism you should pratice in a balanced manner.

Does anyone know what this was? Has there been a retrospective medical diagnosis of this?

From what I have gathered from Zen Sickness this is fatigue due to excessive striving and beyond what the body can bear.

Also possible that this is linked to insight knowledges.

Are there equivalent stages of the path in other traditions?

Another way these symptoms can have arisen is higher insights: The Progress of Insight - Awareness of Fearfulness to Knowledge of Re-observation

Also some mild forms of restlessness can be due to low level defilements and excessive thinking and pondering about the Dhamma which is mentioned in (Yuga,naddha) Paṭipadā Sutta.

Should I be worried?

Yes if it is excessive striving. No if it insight knowledges.

The are few borderlines between mystical (enlightenment) and psychotic experiences.

Read 'Psychotic and Mystical States of Being: Connections and Distinctions', the scholarly paper by Caroline Brett, PhD, in the journal of Philosophy, Psychiatry, & Psychology.

The paper is available free on this website http://www.yogapsychology.org/art_psychoticandmystical.html. Or, if you have access to journals through your library you can download a PDF of Brett's article.)

Brett argues and provides significant evidence that psychotic and mystical experiences (enlightenment states of mind) are fundamentally indistinguishable. She gives some critical distinctions:

"...the psychotic invests the conceptualization of the experience with great emotional significance, because it serves the purpose of contextualizing and reifying the sense of identity, which is under threat."

Before enlightenment: chop wood, carry water; after enlightenment: chop wood, carry water. (Zen proverb)

Hakuin describes it himself in 'Yasenkanna' - the problem seems to have been that his focus was too cerebral, and the remedy given by the sennin was to direct his energy to his lower body.

https://terebess.hu/zen/Yasenkanna2.pdf

I think it's basically impossible to know. According to this page, Hakuin, described it as such:

my heart fire began to rise up­ward against the natural course, parching my lungs of their essen­tial fluids. My feet and legs were always ice-cold: they felt as though they were immersed in tubs of snow. There was a constant buzzing in my ears, as if I were walking beside a raging mountain torrent. I became abnormally weak and timid, shrinking and fear­ful in whatever I did. I felt totally drained, physically and mentally exhausted. Strange visions appeared to me during waking and sleeping hours alike. My armpits were always wet with perspira­tion. My eyes watered constantly.

The article includes a footnote explaining some of the context of such claims about "heart fire":

This was a basic notion in Chinese medical lore. Cf. the statement in the encyclopaedic compilation Wu tsa tsu (Five Assorted Offerings, the section on “Man”), by the Ming scholar Hsieh Chao-che: “When a person is engaged in too much intellection, the heart fire burns excessively and mounts upward.”

Torei’s Biography (1710, Age 25) lists twelve morbid symptoms that appeared: firelike burning in the head; loins and legs ice-cold; eyes constantly watering; ringing in the ears; instinctive shrinking from sunlight; irrepressible sadness in darkness or shade; thinking an intolerable burden; recurrent bad dreams sapping his strength; emission of s*m*n during sleep; restlessness and nervousness during waking hours; difficulty digesting food; cold chills unrelieved by heavy clothing.

He lists very specific ailments that are difficult to tie to particular problems connected with meditation practice, and without further accounts from others claiming to have the same thing, how can we know?

Willoughby Britton at Brown University is investigating problems that are arising in people following Mindfulness at the moment, and it's possible that Hakuin's overzealous nature led him to experience what he did (as her research seems to indicate a connection with overly intense practice).

You can read about it here: The Dark Night of the Soul

I would be worried. Happened to me and many other intensive meditators. "Meditation sickness" occurs to those meditators embedded in an ideology or worldview that values silence and stilling thought as superior or as more valuable than thinking or acting. This devalues thought, thinking, and acting in the world. And, these value/devalues are what is associated with "meditation sickness".

Celebrated Zen master Guiding Zongmi, warned that excessive focus on meditation and achieving "inner stillness, and non-critical/non-analytical study of the scriptures led to meditation sickness.

Robert H. Sharf, Professor of Buddhist Studies in the Department of East Asian Languages and Cultures, as well as Chair of the Center for Buddhist Studies, at the University of California, Berkeley, wrote of meditation sickness:

"Meditation sickness" excessive focus in meditation on achieving “inner stillness” (ningji), especially when unbalanced by an engagement with the scriptures, leads to a state described as “falling into emptiness” (duokong), which is, in turn, associated with “meditation sickness” (changing). The term meditation sickness was used by various Buddhist masters as a critique of practices they deemed detrimental to the path, notably techniques that emphasized inner stillness—they seem to have been targeting practices that cultivated a sort of non-critical or non-analytical presentness." -- from p476 Is mindfulness Buddhist? (and why it matters), Robert H. Sharf, University of California, Berkeley, Transcultural Psychiatry 2015, Vol. 52(4) 470–484