Which of the following is considered by many to be the biggest advancement in sleep study?

Discovery of Rapid Eye Movement

Electroencephalograph (EEG)

Sleep cycles

Discovery of Circadian rhythm

I think the correct answer from the choices listed above is option A. It is the discovery of rapid eye movement that is considered by many to be the biggest advancement in sleep study. Nathaniel Kleitman, and William C. Dement, discovered phases of rapid eye movement during sleep, and connected these to dreaming.