Here’s a question that was directed to me from someone on “The Dao Bums” forum site, and my reply:
Do you know any reason why I am getting a headache after meditation? I can’t sleep well if I meditate more than 20 minutes.
I have no experience with either of those symptoms.
Something I wrote might apply:
In one of his lectures, Shunryu Suzuki spoke about the difference between “preparatory practice” and “shikantaza”, or “just sitting”:
But usually in counting breathing or following breathing, you feel as if you are doing something, you know– you are following breathing, and you are counting breathing. This is, you know, why counting breathing or following breathing practice is, you know, for us it is some preparation– preparatory practice for shikantaza because for most people it is rather difficult to sit, you know, just to sit.
(“The Background of Shikantaza”, Shunryu Suzuki; San Francisco, February 22, 1970)
Suzuki said that directing attention to the movement of breath (“following breathing… counting breathing”) has the feeling of “doing something”, and that “doing something” makes such practice only preparatory.
Although attention can be directed to the movement of breath, necessity in the movement of breath can also direct attention:
There can… come a moment when the movement of breath necessitates the placement of attention at a certain location in the body, or at a series of locations, with the ability to remain awake as the location of attention shifts retained through the exercise of presence.
There’s a frailty in the structure of the lower spine, and the movement of breath can place the point of awareness in such a fashion as to engage a mechanism of support for the spine, often in stages.
(Shunryu Suzuki on Shikantaza and the Theravadin Stages)
The idea here is that the consciousness identified with the self, with “I am”, must be allowed to move freely in the body, while a presence of mind sufficient to retain one-pointedness is mustered up. If the presence is mustered, there’s a feeling of ease connected with one-pointedness, and a sense of gravity (“towering up like a mile-high wall”, as Yuanwu used to say) that allows the automatic activity of the body to proceed solely by virtue of one-pointedness (and the location of one-pointedness).
If the placement of attention is not through a necessity of breath or posture (or more) that comes in the course of an inhalation or exhalation, then the automatic activity of the body is inhibited. Maybe that could result in a headache, but I’m guessing, as that’s not a symptom I’ve experienced.
Ta-chi said, “How can you produce a mirror by polishing a tile?”
Nan-yueh replied, “How can you make a Buddha by sitting in meditation?”
Ta-chi asked, “Then, what is right?”
Nan-yueh answered, “When a man is driving a cart, if the cart doesn’t go, should he beat the cart or beat the ox?”
(“Dogen’s Manuals of Zen Meditation”, Carl Bielefeldt, p 195-195, UC Press ed. 1988)
My advice would be, keep a presence of mind with the location of consciousness right before you fall asleep–see where the mind goes.