Doxylamine
"This is the first time I took unisom. I'm pregnant and have been having trouble sleeping. I took it the first night and I as asleep within 30 minutes. It was the best sleep I've had in awhile and woke up feeling great but the second night taking it didn't go so well. It didn't make me drowsy one bit. I tried taking a second pill and nothing worked. I'm not sure why it worked the first night for me but not the second. :("