Low doses of the toxic gas responsible for the unpleasant odor of rotten eggs can safely and reversibly depress both metabolism and aspects of cardiovascular function in mice, producing a suspended-animation-like state. In the April 2008 issue of the journal Anesthesiology, Massachusetts General Hospital (MGH) reseachers report that effects seen in earlier studies of hydrogen sulfide do not depend on a reduction in body temperature and include a substantial decrease in heart rate without a drop in blood pressure.
I seem to remember this "discovery" cropping up a few years ago. Fortean researchers were quick to note the presence of sulfurous odors in the vicinity of UFOs and perceived aliens, offering the possibility that "abductions" were accomplished with the assistance of hydrogen sulfide "anesthesia." It strikes me as a messy way for members of a presumably advanced civilization to go about immobilizing witnesses, but it can't be immediately discounted. Perhaps a more likely explanation is that exposure to strong EM fields can trip hallucinatory triggers in the brain, as argued by Michael Persinger and Albert Budden.
An engaging question then arises: are the "aliens" (and their craft) the source of the EM radiation or a by-product?