My daughter is 4 1/2 months old and she only naps for 30-45 minutes at a time. She wakes up happy, but she is tired again after a little over an hour. 

I know some babies are naturally short nappers. But there have been times where she will sleep two hours or longer if I'm holding her (which I have stopped doing altogether for her naps). 

I have a short nap routine and I use a sound machine during her naps. 

Any advice? Thanks in advance! 

-- She takes 4 or 5 naps per day. She is only awake 1.5 hours before she begins showing sleep signs.