My daughter seems to be able to sleep about 30 seconds after laying down.  Best I can remember, her mother is the same way.  For me, it's more like 20 minutes to an hour and I've always been a little jealous of people who seem to be able to just flip a switch and be out for the night.

I recently learned that it's actually not a great sign if you fall asleep that fast.  It might indicate that you're generally over tired or maybe that the sleep you get isn't that great.  I don't think that is the case with my kid.  I think, like her mother, she is just capable of sleeping as much as the average housecat if given the chance.

There are some other "sleep myths" that this article can give you the truth about.