Daytime sleepiness, particularly in the first trimester, encourages almost every pregnant woman to nap. But naps make it more difficult to sleep at night because they take away some of the sleep pressure that builds up over the day.

"It's like snacking," says Grace Pien, M.D., a sleep researcher at the University of Pennsylvania's Center for Sleep and Respiratory Neurobiology. "Eating even a small amount of food before a meal can take away your appetite."