Dreams lasting 20–40 minutes: normal or problematic?

You put the baby down for a nap, took a deep breath, poured yourself a cup of hot tea, brought a sandwich/cookie/meatball to your mouth — and then heard crying... No, you’re not imagining it. No, the sound isn't coming from outside. Yes, it’s your child crying, having woken up 20/30/40 minutes after falling asleep. What does this mean and what can you do about it?