This is an intriguing comment, but I'm not sure it's valid in all cases. From my experience, getting up to feed a baby isn't something a woman does to fulfill h

This is an intriguing comment, but I'm not sure it's valid in all cases. From my experience, getting up to feed a baby isn't something a woman does to fulfill her role - it's something she does to get sleep. I could not sleep if my baby was crying, or even just restless. My husband could. I could feed the baby; my husband could not. So I was the one who woke up to feed the baby. (Fortunately, feeding him put me right back to sleep, most of the time, due to the drowsiness-inducing effects of oxytocin.) I get the same kind of elevated mood from staying up all night programming or writing, not because it fulfills my role as I believe it to be - though it does, at least when it goes well - but because there is a physiological chain of events that leads to the mood elevation. I'm glad researchers are working on figuring out exactly what that chain of events is, because as a lifelong insomniac, it's a phenomenon I've noticed before, and I've often wished I could bottle it.