There are many forms of dark nights.
In general, a dark night happens when life strongly challenges what we hold as true about ourselves and the world. That’s why the term “dark night” is sometimes used about ordinary challenging life situations, and sometimes in a more technical sense about parts of a spiritual path and process.
One type of a spiritual dark night can happen following an initial opening or awakening. With a spiritual opening, there is an opening to all as Spirit, and sometimes also an opening to previously unprocessed psychological material. As Adyashanti says, the lid is taken off and it comes to the surface. And that can be challenging in the best of circumstances.
In this type of dark night, we find ourselves in a difficult combination of (a) unprocessed psychological material coming to the surface, and (b) being unable to consistently see through it, or see it for what it is, since our clarity is not yet thoroughly clear or stable. There can be a great deal of difficult material coming up, from this and perhaps past lives. And since we are still “baby Buddhas” our clarity hasn’t deepened or matured sufficiently for us to clearly see everything for what it is. We still get caught in what comes up, at least at times.
As Evelyn Underhill and others point out, this is a deeply human process. It can be humanizing. Humbling. Painful. And it’s a pretty ordinary part of the process for many people. And we do get through it, whether we fight it (painful) or learn to go along with it (a little easier). It’s what I went through for a few years after the second opening (relative nondual clarity for about six months).
See below for a more detailed initial draft.