When we put Dale into her crib for nap and night time, or when she's tired, she does this super cute little thing.  With one hand she flops her arm over her face and with the other she caresses her ear.  She then usually starts licking her bicep while her eyes get droopy.  It's about the cutest thing ever.   So when she's tired we just say, "Dale, lick your bicep.  It'll make you feel better."