The role of wake "intensity" in sleep regulation has been put forward more than 40 years ago in the frame of the "homeostasis model of delta sleep" (Feinberg, 1974). According to this model, sleep homeostasis would be determined not only by wake duration but also by the intensity of waking brain activity, which in turn depends on the quantity and quality of physical and cognitive activity carried out during wake. This idea received initial support from animal studies showing increases in slow-wave sleep (SWS) after experimental manipulations of cerebral metabolic rate during wakefulness (