the 40 minute nap

Noah is king of the 40 minute nap.  Without fail, he wakes up and starts to fuss after exactly 40 minutes.  What gives?  How do I get him to sleep for longer stretches at nap times?  Pete thinks it’s the sleep sheep (which coincidentally stops the white noise after 45 minutes), but Noah has done the same thing even without the sleep sheep on!  Yes, there have been longer naps randomly here and there, but on the whole the 40 minute nap is usually the only time off I get during the day and it is spent either 1) eating, 2) showering and drying my hair, or 3) emailing.  Right now he is taking 3 naps per day for 40 minutes each time (which I’m sure will change as things change daily with him!).