Final answer:
The definition of sleepiness according to the materials provided is related to a physiological need for sleep, which ties closely to the concept of 'sleep rebound' that occurs after sleep deprivation.
Step-by-step explanation:
The definition of sleepiness according to Dinges et al. 1987 is not explicitly mentioned in the information provided. However, based on the given material, sleepiness can generally be associated with a physiological need for sleep. Sleep involves low levels of physical activity and reduced sensory awareness, regulated by circadian and homeostatic mechanisms. When an individual is deprived of sleep, there is a phenomenon called sleep rebound, where a person will fall asleep more quickly during subsequent opportunities for sleep, indicating a homeostatic drive to restore balance in sleep patterns. Therefore, while Dinges et al.'s exact definition isn't provided here, based on the context, the closest answer would be (b) A physiological need for sleep.