What I'm curious about, however, is the spiritual dimension to mania or even psychosis. What kind of link exists here?
Neuro-biologists are searching for this answer as well, from a scientific standpoint. They have pretty concrete explanations for how periods of mental illness occur, as well as explanations for the occurrence of religious experience. All are somatic in nature; that is, the brain is the seat of experiential reality. What goes on in the brain neurologically determines our reality. According to this view, all religious visions (visual, auditory, or otherwise) of a supernatural being are the result of physical manipulation of receptors, chemicals, etc., within the brain. What appears "real" to us is only a hologram created by the mind.
In this explanation human beings are 'closed systems'--all experience arises from the mind (as buddhists might say). Scientists study the physical vessel of the mind--the brain--and are able to chart neurological response through instrumentation.
Theistic explanations vary in the sense that they affirm the possibility of an external manipulation of the somatic mind. Science cannot explain why exactly mania, depression, and psychosis occur. For this reason it should be considered as a possibility that a God unbounded by limitation might manipulate our senses in order to communicate a reality that cannot be transmitted in our "normal" plane of existence. This does not deny that apparitions may be the result of brain chemistry. The question to be asked is how or why such a manipulation occurred in the first place. Nothing is random in spiritual thinking; nothing should be thrown out.
Are we better able to access a spiritual realm during mania/hypomania and/or psychosis?
I don't think periods of mental illness necessarily grant access to the spiritual realm. It is true, however, that the spiritual lies beyond the realm of our natural reasoning. Mental illness by nature takes one out of such "normalcy" and places a person in an absurd, non-rational world that can often times be quite frightening. In the sense that mental illness takes us out of our sternly rational way of thinking, it opens one up to the possibility of alternate realities, the spiritual being one of them.
Why do some feel a feeling of universal oneness during those periods?
Again, science can explain this one from a neurobiological perspective: "feelings of universal oneness" are just one of the many sensations the brain creates through its neurological processes. When we "feel" sad, it is our brain creating the chemistry that makes our conscious emotional self "feel." Bodily responses will often follow...tears, cries, emptiness. Without a brain we would have no feelings, so it is not uncommon (again, in Buddhism) to speak of the Mind and the Heart interchangeably.
Can we get there without mania?
Yes, and this is the purpose of spiritual practice and religious discipline. Mania is not dissimilar to a drug. You can use psychoactive substances to manipulate your own brain and induce spiritual experiences. Many religious cultures do this with herbs, plants, etc. But I like to think of our authentic Mind as existing beyond a finite substance. Drugs are often used as a shortcut to spiritual enlightenment, but I think that is erroneous thinking. People who attach too strongly to their mental illness run the same risk.
Is it real?
Not everything experienced in authentic or "real." I think it is safer to say that things experienced in states of mania are not necessarily un-real. They are certainly fantastic and extra-ordinary. But then again, so is God. And these periods do not last. Like feelings, they are transitory. So if you put too much stock in the "realness" of a manic episode, you will be left high and dry when it ends. Mania is not necessarily real, and not necessarily unreal. One should take it for what it is.