great question

oxytocin levels are at their (blood serum) highest levels between the hours of 2-4am...and oxytocin is the hormone that promotes uterine contractions

also the tendency for us to go into labor at night is also based in our primitive ancestory...where it was safer to birth at night

also we are at our most relaxed at night...no adrenaline to suppress our oxytocin levels

xx yogababy