The long dark night of the soul, as described by John of the Cross, is challenging to understand. Here is how I understand it, although I do not claim to fully understand it. At first glance it seems like depression but that is not an explanation. Psychologists understand very little about what depression is, other than that it exists as a fuzzy cluster of symptoms and that there are ways to treat it. But attaching a label to something does not explain "what" it is, just "that" it is. William James, in his classic book, the varieties of religious experience, noted that people who have deep religious experiences tend to arrive at the experience from either a positive, sunny place or a dark, empty place. James's opinion was that there is something deeper in the experience of struggling through darkness to light, than in going from well adjusted to light. However, since James himself arrived from a dark place his opinion may be biased. The dark night of the soul is more specific though. It refers a situation in which an individual has managed to become close to God and experience great joy and great peace - then this experience disappears. Essentially, they become normal again, but now normality seems painfully empty. From a psychological perspective, this makes sense. There is a lot research indicating that happiness is relative to a kind of running average of our experience. This is why people who win the lottery and people who become paralyzed often attain the same level of happiness they had before, after some time has passed. In other words, happiness is relative to what we normally experience; if things are better than normal we are happy and if they are worse than normal we are sad. So blissful experiences raise the average quite a bit and the removal of these therefore leads to a deeply unsatisfying state. However, note that if the bliss continues it becomes less special, it becomes normal. St. John prescribes what to do if you find yourself in the dark night. His answer is to be in it. Don't try to escape it, because you cannot will yourself out of it, nor can you bargain yourself out of it. You need to accept it as where you are now and sit in it in complete silence. This silence is a type of faith, or fearlessness (as the shambala school of Buddhism would call it). From this state of deep emptiness, devoid of any feeling, will arise the dawning of something wonderful, like a rising sun (again, a shambala metaphor). This dawning comes of its own accord, we just wait patiently and fearlessly for it. It can come fast or it can take a long time, but the lesson from st. John is that the state of waiting is itself, a wonderful state, possibly deeper and more meaningful than the dawning sun. So here is the synopsis - extended periods of blissful/ecstatic experiences can be followed by the dark night of the soul (e.g., john of the cross, mother theresa). However, it is possible to go through the dark night of the soul to a deeper way of experiencing God, one that is not relative to our normal experience. From a Buddhist perspective we would call this the Buddha nature and it is believed to exist in all people. From reading different accounts of religious experience, both Christian and Buddhist, it is clear that it is not necessary to go through the dark night to get to this place, but for some people it may be necessary. But even for those who have achieved this state, it is a constant matter of practice and refinement, and the dark night can occur at any time, especially If you are in alienating or difficult circumstance. So it is good to know what to do. It is interesting to note that the dark night of the soul phenomena is discussed a lot more in Christianity than in Buddhism. I think there are several possible reasons for this. One is that the practice of Christianity may encourage this route more than the practices of Buddhism. Also, Buddhist practice prescribes exactly the right thing to do if this happens, so practicing Buddhists may be less freaked out and confused by the experience. Finally, trying to understand this through Christian theology, in my opinion, is likely to be a gigantic mistake. It is the mystical side of Christianity, such as john of the cross, where the answer is found.