Benadryl (found in many over-the-counter sleep aids) and Ambien, are classified as Class B medications.

http://www.babycenter.com/406_is-it-safe-to-take-sleeping-pills-during-pregnancy_1246859.bc

Several medications, including Benadryl (found in many over-the-counter sleep aids) and Ambien, are classified as Class B medications, which means they're presumed safe for pregnant women based on animal studies, but no well-controlled human studies are available.