The Finding That Sleep Becomes More Efficient in People Who

The finding that sleep becomes more efficient in people who regularly sleep less is a key finding because it indicates that

A) sleep-deprivation studies can indicate how much sleep people need only if the volunteers who are being studied are efficient sleepers.
B) sleep is a positive incentive.
C) sleep can be made more efficient by reinforcements and punishments.
D) only circadian theories can account for the negative effects of sleep deprivation.
E) sleep can be conditioned.