My daughter Vanessa started going through a period when she hit 5 months where she would get up in the middle of the night and cry.  She wasn't hungry and was still very tired so after a snuggle she would go right back to sleep.  It wasn't ever on a consistant basis and the last time we were at the health nurses office I asked if they had any suggestions.  The nurse I spoke with said that babies from four months on can start to have nightmares and so just like anyone having a bad dream they need to be reasured.  She suggested that I put a shirt of mine that I wore that day in her crib with her (so long as it didn't have any buttons or small things the baby could choke on etc) so that night I did it.  That night I woke up around 3 AM and did my usual check in on her and to my surprise and delight there she was all snuggled up to my shirt out cold.  Since that night I have consistantly put in a shirt of mine in her crib with her.