Mark's mother is worried because he only sleeps about 4 to 5 hours per night. What would you tell her?

a. She has reason to be concerned because, in the long run, Mark's lack of sleep will take its toll physically.
b. There is nothing to worry about because young people need less sleep than older people.
c. There is no problem as long as Mark spends most of his sleep time in REM sleep.
d. There is probably nothing to be worried about since different people need differing amounts of sleep.

D