Which of the following is true of sleep deprivation?

A) It is a problem that is declining in intensity in the United States.
B) It is defined as sleeping nine hours or less per day.
C) Over half of Americans say they chronically feel sleep deprived.
D) Middle-aged adults who sleep less show less development of heart disease over time.

According to studies, over half of Americans report feeling chronically sleep deprived. Option A is incorrect as sleep deprivation is not decreasing. Option B is incorrect because the definition of sleep deprivation varies from person to person, and there is no universally agreed-upon definition. Option D is incorrect because sleep deprivation has been linked to cardiovascular disease, not less development of heart disease.