It's an odd thing, but often I feel I have 2 quite distinct versions of the Dark Night -- one where there is headache and heart thumping and ugly sensations, sometimes quite deep in the background if my 3rd jhana is strong enough. And one where it's all fast and fine streaming vibrations but with a tense, edgy quality to them.
It's possible, even likely, that this last is not DN at all but Equanimity. I've heard others describing the higher end of this nana as being a bit scary. Maybe that's it. This stuff used to be really easy diagnose, but these days its all fuzzy and less well defined.
Today I've sat 2 x 1hr sits. The last dropped me into 4th jhana fairly quickly. I have begun to recognise the sinking / depth kind of feeling as the envelopment of 4th -- I didn't really understand what I was experiencing until I saw this rather
excellent talk by Ayya Khema describing 4th as having the quality of
depth. That's certainly what I experience. It's like lying on the bottom of a very deep, still lake.
After some fairly dreary minutes filled with pretty regular vibrations there was an awful lot of "zooming" in, as vibrations got faster and finer. But they came with some scariness, a kind of "im going to implode!" feeling, a feeling of not being able to take any more. I've learnt to relax into it and take more, and more. Im not sure if this is good or not, but it seems to be the right direction.
At times it seems like I can sense the vibrations / impermanence of my body slotting into the impermanence of sounds out of the street, of the feeling of the bed Im lying on, like although i know roughly where my body parts are, they are only held loosely together. And that I am only a twitch of perspective away from expanding into the whole room (or fading into it). I haven't quite managed this yet but it seems like it may morph toward the 5th jhana sometimes when this happens.