It is not uncommon for non terrestrial beings to cause soul leaking vampirism. Sleep works most of the time, but due to the fact that there is increased renal activity, the patient cannot sleep, as he has to go for a piss every half an hour or so. When the renal activity stops, it is a good sign that the soul leaking has stopped. Not all dreaming can be beneficial. Some dreaming can be distasteful and painful, and the subject may awake with further leaks.
Smoking helps a lot.
There seems to be some sort of leak-patching done with nicotine that helps the subject avoid the chest pains. Of course by the time the disease sets, the patient won't give a rat's ass about the ill effects of smoking. Plain termination is much more favorable to Soul Cancer anyway. If they see that, they won't spend time with you, because you have no soul energy to spare. They usually go after healthy ones, who can provide them with good amounts of energy.