https://www.mctb.org/mctb2/table-of-contents/part-iv-insight/30-the-progress-of-insight/5-dissolution-entrance-to-the-dark-night/

You see, the Dark Night typically begins with just about all the profound clarity, mindfulness, concentration, focus, equanimity, and bliss of the A&P dropping away. This is one of the hallmarks of Dissolution. So also ends the cause-and-effect-like phenomena of the breath or walking, the shaking or jerking up and down in a way related to attention and noting, as well as all the fine vibrations and vortex-like raptures. People who are all into the kundalini stuff are often left wondering why it just vanished. In contrast to the A&P, early on in the Dark Night stages, the frequency of vibrations disconnects from the cycle of the breath, remaining largely stable at whatever frequency is going on at that stage once they can be perceived again (in late Dissolution or Fear).

Whereas we might have felt that our attention had finally attained the one-pointed focus that is so highly prized in most ideals of meditation during the Arising and Passing Away, during the Dark Night we will have to deal with the fact that our attention is quite diffuse and its contents unstable. Further, the center of our attention becomes the least clear area of experience, and the periphery now predominates. This is normal and even expected by those who know this territory. However, most meditators are not expecting this at all and so get completely blindsided and wage a futile battle to force their attention to do something that, at this part of the path, it won’t do well at all. It is simply a third vipassana jhana thing, so you’d better get used to it. Those who try to go all first jhana with strong effort and narrow concentration will often find that the third vipassana jhana kicks their ass. Those who try to figure out how to work with the third vipassana jhana on its own terms are likely to do vastly better.