How do you shut off their brain?

Last night we (me and the little ones) started a cooking project (monkey bread) later than we should have. After disolving and stiring and raising and kneeding and cutting and rolling and stacking it was much later than we should have been up on a school night, but how do you stop right in the middle? Needless to say, I was anxious to get them quickly into the bath and then bed - not to mention hair, teeth, pjs, books, and prayers. The lights were off and heads were on pillows and I hear "b-i-n-g-o" then two seconds later, "your friends are my friends" followed closely by "looby loo". Why is it that late at night their little minds open and everything flows out. They want to chat and sing and recite newly found facts and ask questions. Kids are amazing, but how do you get them to shut down?