I suspect that evolution has programmed us to seek out a mate with the opposite sleep tendencies of our own, so that one half of the partnership can wake up at the smallest twig cracking and protect the family from sabretooth tigers, etc, particularly during the child-rearing years, and the other partner can sleep like the dead and get a good night’s sleep to be happy, healthy, and live into old age.
Certainly, I’ve always been the former while my wife has been the latter. When we were young and parenting only a small dog, she knew exactly who you come to in the middle of the night to go out to potty, because it was only possible to awaken one of us.
And that’s why I’ve been awake since 6:30 on a Saturday, because our hungry-hungry six-year-old also knows who to wake up.
There was a brief period in our relationship when this dynamic flipped: when any of our children were still breastfeeding. Then and only then my wife was on high alert to the needs of another living thing (a living thing that alternated between sharing our bed and sleeping right next to it). For that all-too-brief period, I was allowed to sleep pretty well through the night (unless the dog needed me or the baby REALLY started to cry).
But those times are long gone, and none of the routine needs of our children is powerful enough to lift the weighted blanket of sleep from the other side of the bed.