Yep, the longer they sleep at night, the less sleep they need during the day. They still need 2 naps + 1 catnap before 6 months...but if she's getting 12 hours at night, that's only 3 hours in naps, so 3-45 min cycles may be all she needs to be happy.
Also, your 45 min naps may be the cause of daytime sleep developing. This happens around 3-4 months. Just keep trying to push waking time up to 2 hours in 15 min incraments if you haven't already. Until it straightens itself out, you may only see 45 min naps for awhile.
Totally agree though about if they wake happy after 45 mins, it's more likely a biorhythm thing, and there's no reason to extend the nap.
*HUGS*
Sonya =P