Maybe I am concerned for nothing, but I've noticed that my month-old baby alternates sleepy days and awake days. Example: day one, she'll be up for long stretches, uninterested in napping; day two, she'll sleep a lot--and it goes back and forth like that. She still wakes to feed appropriately, when she's awake she's totally alert. Maybe she needs more naps on the awake days so that she's not so sleepy the next, but me being a first-timer, I'm more worried that she's not regulating sleep cycles appropriately and she's stuck on some weird 48-hour-day.
The doctor says as long as she's eating, peeing and pooping, normally, then the baby is fine, but that's basically her answer for everything. I'd feel better if I heard that someone else experienced this, too, and that this really isn't a cause for concern that baby has some kind of sleep disorder.
Re: sleep cycle worry
My first born was just like this. Be up all day one day and sleep away the next. She's now 2.5 and sleeps just fine.
My NB on the other hand..we're still figuring out days and nights.
I wouldn't worry about it.