！
No going back to sleep!
Literal
Second-sleep don't-do-[emphatic-のっ]!
A sharp scolding command. 二度寝 ('second sleep / going back to sleep') is a productive 二度+verb-stem compound describing the act of falling asleep again after initially waking. しないの is the の-question ending used here as an imperative/scolding ('you're not going to, right?'). The small っ adds finality. Efficient spoken Japanese: one compound noun and a compact imperative.