There is a theory that after birth, newborns fall into a deep sleep for the first day or so, only really waking up to feed.
Then around the second night, they start to realise their surroundings have changed - there is no constant heartbeat sound, its much colder, there are no constant movements and the feeling of being held tightly and no constant supply of food.
All of these may contribute to your newborn being a little more unsettled and if you are prepared for it, you know how to respond with lots of close skin to skin cuddling and feeding on demand.
The term syndrome implies that something is wrong but this behaviour is completely normal and most babies experience it, perhaps even on night three or four.
Your baby just needs you and the reassurance that you are still there.