I believe it is because babies are bloody inconvenient at the best of times, and they're getting a head-start on making us lose sleep by sitting on our bladders all night, then making us go into labour in the wee hours just when we're finally getting comfy. I was in labour with mild contractions from about 2pm on a Saturday afternoon, 5cm dilated at around 6pm, went home from hospital and went to bed at 11 feeling quite comfortable, then BANG GTFO OF BED MUM IT'S 3:30AM AND I FEEL LIKE MAKING AN APPEARANCE. :/

Best of luck