There is debate among researchers as to whether the concept of sleep debt describes a measurable phenomenon. The September 2004 issue of the journal Sleep contains dueling editorials from two leading sleep researchers, David F. Dinges[2] and Jim Horne.[3] A 1997 experiment conducted by psychiatrists at the University of Pennsylvania School of Medicine suggested that cumulative nocturnal sleep debt affects daytime sleepiness, particularly on the first, second, sixth, and seventh days of sleep restriction.[4]