Howard is 88 years old and in excellent health. All his life, he has thrived on just 5 hours of sleep a night. The most likely explanation for Howard's good health despite his lifelong short sleep habit is that he...

A. drinks four or more caffeinated beverages per day.
B. is inaccurate in recalling how much sleep he gets.
C. has a unipolar mood disorder that decreases his sleep needs.
D. has a genetic variant affecting his circadian rhythm.