I have done some research online and read articles about zen sickness.
There are a couple of problems when doing research, online, on an ailment which, can be "bearing" on enlightenment, because there are lots of "me too is having this ailment", in other words "I too, am close to being enlightened"; And suddenly, zen sickness symptoms multiplied: psychological depersonalisation, etc.. are linked to zen sickness and the definition of zen sickness becomes vague and fuzzy.
Even when I confine my readings to only very few recorded accomplished zen/chan masters, who actually do intensive meditations, their descriptions of zen sickness differ, and that is perhaps due to incorrect attribution ( the "me too" phenomena) of an experience to zen sickness. (I decided, eventually, not to provide links because it would create more confusion/debate on it, so don't take my word on it).
The zen sickness of Master Han Shan, IMHO, seems to be a genuine description, but I don't agree to some of the discussion, especially the last but one paragraph which begins with:
The situation of Zen sickness is not something you should desire, and certainly not something you want to force into occurring. Since it is a trouble or affliction, you want to avoid it if you can, but the important thing is that there is a remedy if it occurs. Think of it in terms of ....
It is not something you can force or get into easily unless you want to reprogram your own mind, but that's my view.
From my reading and analysis it would correspond to an Anagami in the Theravada tradition working towards Arahatship, where at that stage the most prominent problem is restlessness.
IMHO, at the path stage of an Anagami it has seen emptiness clearly and enjoys the bliss of holding onto emptiness. To get to the fruit of Anagami it has to let go of that hold on emptiness. Once it has decided to let go of emptiness the bliss, vanished. The bliss makes the mind concentrated, I quote "the purpose of bliss is concentration"(.. in some sutta).
With the letting go of emptiness & bliss the mind will just grab at any thought,tune,occupations to concentrate the mind, because if it didn't, the restlessness of the mind is unbearable( we are here talking about a very sensitive fine mind; restlessness is one of the last five fetters ).
Why then can not an Anagami phala(phala sounds better than fruit, which reminds me of fruitcake) go back to holding onto emptiness again and get the bliss back? The answer is that it can't, but I won't go into that.
In Theravada we would call it restlessness rather than a (zen) sickness. In the articles on zen sickness I've read, the recommendation is lots of sleep, or get drowsy with alcohol, if permitted, and then get to sleep. But if restlessness is, like I say, unbearable sleep is not an option.
The answer is learning how to abide in emptiness after letting go of emptiness. How to do that is another topic.
The deep state of samadhi in which Master Han Shan was in for 5 days is, in the Theravada tradition equivalent to sanna-vedayita nirodha ( ceassation of perception and feeling) in which when one awakes, enlightenment is gain (but there are controversies, some say back to Anagami phala if it wasn't attained), I haven't a clue!
So it's a bit of way to go before you encounter it and when you encounter it just learn to abide in emptiness, then the restlessness will not go away but lessen. But if you practice jhannas the mind will lead you into sanna-vedayita nirodha for 7 days, like Master Han Shan. The you can tell us about it!