It is interesting to think that the scientific study of sleep and dreams received a tremendous boost. Aserinsky and Kleitman used an electroencephalograph (EEG) in order to measure their subjects' sleep. Jouvet, a French sleep researcher from the University of Lyon, cuts the descending neural pathways that are responsible for the inhibition of motor movements during rapid eye movements (REM) sleep in cats. After Aserinsky and Kleitman's discovery of a distinct stage of sleep for vivid dreaming, sleep researchers settled down to the present thinking that sleep consists of different stages. The major evolutionary advance in the Homo line, Homo erectus, clearly did give up sleeping in trees. Thomas Wynn and the author suggest that the change from tree to ground sleep was no mere accoutrement. Thus, sleeping on the ground, while it may have released early human types to receive more delta and REM sleep, made them inordinately more susceptible to being preyed upon.