Well, for me, it was always in how my babies woke from their naps. If they took a nap that was on the shorter side, they'd wake up and be WIDE awake -- I would go in to try to shush them back to sleep, or otherwise try to get them back to sleep, and they'd be smiling and giggly and there'd be no hope of more sleep. It can also show when you put them down for a nap and they will play for a long time or sometimes they just get fussy and mad because they still need to be up longer (though that last one can also happen when they're OT)!
