Which element did Dement and Kleitman NOT investigate in their study?

Dement and Kleitman's study primarily focused on the relationship between eye movements and dreaming, as well as the physiological aspects of sleep, including brain activity during various sleep stages. They investigated how rapid eye movement (REM) was associated with dreams and explored the content of dreams as recalled by the participants.

The aspect of the impact of sleep on physical health was not a focus of their research. Instead, their major contributions were in the area of sleep cycles, the characteristics of REM sleep, and how these relate to dream recall and quality. By centering on these psychological and physiological dimensions, the study provided valuable insights into sleep and dreaming without delving into physical health implications.