I wasn't bike commuting when my kids were really small; I started up again when my daughter was a bit under two years of age.

But I've found the twice daily manic bike ride energizes me; I may feel tired when I'm getting on the bike, but I feel great when I get off it.

How much attention kids need, at night, when they should be sleeping, is a cultural norm. If they cry, and you sleep through it, they sooner or later learn not to bother with the crying. But if you're trained --by yourself, or by your spouse, or by your idea of what's expected of you-- to get up and deal with the baby every time s/he cries, then that will be expected of you. What I'm driving at is... go ahead and ride your bike. If it makes you more tired, so be it. Maybe you'll sleep better at night, and miss a feeding or two... but it won't be the end of the world. If you feel better, and are happier in general, your whole family will benefit.

How's that for rationalization?