It is possible to faint while you are asleep, although it is extremely rare. Krediet et al. in a 2004 study published in Heart looked at vasovagal syncope interrupting sleep, otherwise known as fainting while asleep, in an attempt to find the reason.
Most of the 13 participants with this issue in the study were women. Of those studied, all had episodes of unconsciousness that interrupted their sleep, notes Krediet et al. Being unconscious while asleep seems like a paradox, but the researchers were able to make their determinations with the results of an electroencephalogram. An EEG, as it is more commonly known, measures and records the electrical activity of the brain. A head-up tilt table test diagnoses the cause of fainting spells. The findings show these were not seizures in sleep but actual fainting. Although two of the participants suffered from epilepsy, this was not seen as the cause for the episodes. All of the patients also suffered from fainting spells during the day.
While many cases of vasovagal syncope are related to heart function abnormalities, Marrison and Perry report in Europace that this is not the case for those who faint while lying down or asleep. The reasons for fainting during sleep are still unknown and being studied.