Teacher voice. It's rest time, lay down.
You could use OTHER times to explain the importance of rest. My own crew have (and always have) responded well to me explaining that the only time you grow is when you're sleeping. I'm pretty short, (about 5' even) so I tell them I didn't sleep/nap enough when I was little. They all WANT to be big, so this works great. Only, that's a discussion for lunch time, and then "Hurry up and sleep, so you can grow" and I always make a point to make comments when they wake up.
"Did you grow? Oh my goodness you're TALLER!" now and then, too.
