When People Go Without Sleep for More Than 24 Hours

When people go without sleep for more than 24 hours in a row, how does their experience of sleepiness vary over time?

A) It increases for a while and then becomes constant.
B) It increases during the night but decreases the next morning.
C) It continues to increase steadily throughout the period when the person does not sleep.
D) It increases and decreases on an irregular cycle that averages about 120 minutes.