nocturnal days

For the past three nights, KC has been waking up between midnight and 2 AM and then staying up until 5 or 6 AM. We can't seem to figure it out. Is he cold, hot, hungry, does he go to bed too late, too early, does he have allergies or something to cause difficult breathing, should I not let him sleep in after a long night of no sleep? I don't know. It seems we've tried different things and he continues to wake up at night. Something must be done. It's like I have a new baby already and I'm in a daze all day long. It looks like we're in for twice the fun in a few months.