When People Go Without Sleep for Several Days in a Row

When people go without sleep for several days in a row, how does their sleepiness change?

A) They feel sleepier and sleepier as their deprivation continues.
B) After one or two days they reach a constant level of sleepiness, with no further change noticeable.
C) For about the first two days they get sleepier, and then they gradually start feeling better.
D) They feel sleepy at night, then less sleepy in the day, and then sleepier again the next night.