Minor & Major Dark Nights
Reading Ken Wilber's article on the Spectrum of Pathologies (in Paths Beyond the Ego) helped me see how the dark night of the soul comes in minor and major varieties.
Minor dark nights
Any time we have an opening experience and it fades (at will do for a while until it becomes more stable), we will experience a dark night of the soul to the extent that we are attached to the experience. These are the more minor forms of the dark night, although I am sure they can be experienced as serious enough.
The minor and medium dark nights tend to come at the beginning and intermediate phases of practice, and at the first and second of the five phases of Tozan & Underhill's framework.
Major dark nights
The major dark night of the soul is the same, although towards the other end of the scale. This one comes after a long lasting and apparently stable and clear awakening, and so tends to be deeper and last longer as well.
It may also be that this one has elements of exhaustion in it. After all, it comes following a period where typically tremendous energies floods through our human self. We are tapped into the source, so we may not really notice, but something is happening on a human level that may lead to collapse and (temporary and apparent) withdrawal from the source.
The other aspect of it may be the need for detachment from our identity with the absolute. As with any exclusive identity, it is limited and limiting, and life leads us into a disidentification so we can find a more inclusive identity. Eventually, when we finally come to a point where we can embrace the dark night, we gradually emerge into a new integration. It all comes back in a new way, with a sense of ordinariness about it, and through deepening integration of the absolute and relative, our human self and Big Mind/Heart.