People from many cultures report being abducted in their sleep by a range of nocturnal spirit creatures, from the Grim Reaper to teams of babies. Quentin Cooper talks to Chris French, head of Anomalistic Psychology Research Unit at Goldsmith's College, London. His research suggests that alien abduction experiences are similar to other paranormal phenomena, such as ghosts, and that their physical grounding may be in sleep paralysis. Producer Julian Siddle