I have heard a theory that women go into Labor at night more often as it's nature's way of there being a better chance that DH is around. It's an interesting theory but there's probably not truth to it. I went into Labour at around 9pm with DS1 and around 3pm with DS2 (and that was a 3 hr labour), so I've had one of each. I think it could happen at any time for you - maybe you can update in here when it happens and we can see whether it was true for you or not?