Napping trouble

My one month old only takes 30 minute naps! She sleeps great at night and has since her second week home - usually 3-5 hour stretches, wakes to eat, then hoes back to sleep. However, during the day I can't get her to nap more than 30 minutes at a time. She wakes up at 30 minutes  almost on the dot. We have a sound machine in the bedroom, so nothing is startling her awake. Any help or insight would be great. She had been doing this for a few weeks now at least. Oh, and we don't swaddled her. We stopped after the first week because we figured out she likes to sleep with her hands on her face. She sleeps great at night without a saddle, so I wouldn't figure daytime to be any different.