It's not true polyphasic sleep; it's just the best way to deal safely with exhaustion. And just about every new parent goes through a period of exhaustion, because the baby will probably be waking them during their normal sleeping hours, every night... plus there's more to do during the day, so especially first time parents start out trying to do it all (but without the sleep to recover at night), and that catches up with you pretty quickly.

Plus the experience of giving birth to launch this whole adventure means that probably both parents went for a few days with no or very little sleep, and the mother has a lot of physical recovery to do as well (ever worse if it was a c-section), so they're starting off way in the red.