Werewolves   aren't the only creatures affected by the moon's cycles . A full moon also affects people's sleep more or less, reports a Swiss team of scientists. Even people who sleep in a lab without windows could reduce their sleeping time a little once a month. To test the moon's influence, 33 adults of both sexes and ages of many kinds 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 sleepers took an average of five minutes longer to fall asleep, and slept for 20 minutes less. In addition, brain activity fell by 30 percent during the sleep. And hormone levels that help control sleep cycles went down. On these nights, the sleepers complained they couldn't sleep well even though they didn't know the moon's cycle. On the bright side, no sleeper 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 affected by the moon cycles. There may be another reason, 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. Those sleepers could have been affected by having too much 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 influence on sleeping and its possible reasons the ways to get a better sleep on a full moon night