This happened with DS2. I don't know what caused it. He also did it for a few days after he was born. The midwife whitnessed it on a vist and said if he did it again he would have to go into hostpital as it was due to him having a low blood sugar level (I think) I wasn't bothered, as I knew it was something he's always done. Like hell were we going back to hostpital!
Sorry no idea what causes it. Could it actually be beacause of low blood sugar at 4am? Sounds possible, but I'm not a medic.