From Mindfulness in Plain English by Bhante Henepola Gunaratana, Chapter 7 ("What to Do with Your Mind"):
Somewhere in this process, you will come face-to-face with the sudden and shocking realization that you are completely crazy. Your mind is a shrieking, gibbering madhouse on wheels barreling pell-mell down the hill, utterly out of control and hopeless. No problem. You are not crazier than you were yesterday. It has always been this way, and you just never noticed. You are also no crazier than everybody else around you. The only real difference is that you have confronted the situation; they have not. So they still feel relatively comfortable. That does not mean that they are better off. Ignorance may be bliss, but it does not lead to liberation. So don't let this realization unsettle you. It is a milestone actually, a sign of real progress. The very fact that you have looked at the problem straight in the eye means that you are on your way up and out of it.
In the wordless observation of the breath, there are two states to be avoided: thinking and sinking. The thinking mind manifests most clearly as the monkey-mind phenomenon we have just been discussing. The sinking mind is almost the reverse. As a general term, sinking mind denotes any dimming of awareness. At its best, it is sort of a mental vacuum in which there is no thought, no observation of the breath, no awareness of anything. It is a gap, a formless mental gray area rather like a dreamless sleep. Sinking mind is a void. Avoid it.