Werewolves   aren't the only creatures  affected by the moon's cycles. A full moon slightly affects people's sleep, reports a Swiss team of scientists. Even people's sleep in a lab without windows experienced a small shortfall in sleep once a month. To test the moon's effect, 33 adult volunteers of both sexes and various ages spent several nights in a sleep lab. As they slept, researchers recorded their brain activity, eye movements and hormone   levels. On nights closer to a full moon, the subjects took an average of five minutes longer to fall asleep, and slept for 20 minutes less. In addition, brain activity decreased by 30 percent during the sleep stage that the brain normally uses to recover from its daylight work. And levels of hormone that help control sleep cycles dropped. On these nights, the sleepers complained of poor sleep quality even though they were unaware of the moon's cycle. On the bright side, not a single participant turned into a werewolf. The Swiss team doesn't know how the moon affects sleep. The gravity   of the moon causes ocean tides to rise and fall. But that force is too weak to affect sleep, Gajochen says. He believed some body's biological clock may be tied to the moon cycles. There may be another explanation, says David Dinges. This sleep researcher at the University Of Pennsylvania in Philadelphia says that the body clock is very sensitive to light at night. Volunteer's sleep could have been affected by exposure to extra moonlight before arriving at the lab. The passage is mainly about   _  .
the sleeping problems caused by the moon's cycles the influence of the moon on human sleeping habits the moon's effect on sleeping and its possible reasons the ways to improve sleep quality on a full moon night
Question: Werewolves   aren't the only creatures  affected by the moon's cycles. A full moon slightly affects people's sleep, reports a Swiss team of scientists. Even people's sleep in a lab without windows experienced a small shortfall in sleep once a month. To test the moon's effect, 33 adult volunteers of both sexes and various ages spent several nights in a sleep lab. As they slept, researchers recorded their brain activity, eye movements and hormone   levels. On nights closer to a full moon, the subjects took an average of five minutes longer to fall asleep, and slept for 20 minutes less. In addition, brain activity decreased by 30 percent during the sleep stage that the brain normally uses to recover from its daylight work. And levels of hormone that help control sleep cycles dropped. On these nights, the sleepers complained of poor sleep quality even though they were unaware of the moon's cycle. On the bright side, not a single participant turned into a werewolf. The Swiss team doesn't know how the moon affects sleep. The gravity   of the moon causes ocean tides to rise and fall. But that force is too weak to affect sleep, Gajochen says. He believed some body's biological clock may be tied to the moon cycles. There may be another explanation, says David Dinges. This sleep researcher at the University Of Pennsylvania in Philadelphia says that the body clock is very sensitive to light at night. Volunteer's sleep could have been affected by exposure to extra moonlight before arriving at the lab. The passage is mainly about   _  .
A. the sleeping problems caused by the moon's cycles
B. the influence of the moon on human sleeping habits
C. the moon's effect on sleeping and its possible reasons
D. the ways to improve sleep quality on a full moon night
Answer: