Married men and testosterone.

Harvard Magazine reports:

[Peter B.] Gray studied testosterone in saliva collected from 58 men (48 of them Harvard students) between the ages of 20 and 41. Half were married, and of those, 15 were married with children. He took four saliva samples from each man: two in the morning and two in the evening. The subjects also completed questionnaires about their demographic, marital, and parenting backgrounds. Among other things, the questionnaires asked how much time the men spent with their spouses (instead of hanging out with the guys) on their last day off from work, and measured the effort they expended caring for their children. Analysis showed that marriage, fatherhood, and longer periods spent with wives and children were all linked to lower testosterone levels. Fathers in particular had levels significantly lower than those of unmarried men. Researchers also observed that hormone levels in the morning samples were high and relatively even among the men; the differences appeared at night.

This is all very interesting, but let me extract one very useful piece of advice: Marrieds should not make bed time their primary time for having sex. Quite aside from the fact that you're tired, if this study holds true, the man is likely to have lower testosterone.