One of the definitions of stay is to stop something. So "I can't stay sleep." means "I can't stop sleep". Shakespeare, Romeo and Juliet: V, iii: "..But he whi

One of the definitions of stay is to stop something. So "I can't stay sleep." means "I can't stop sleep". Shakespeare, Romeo and Juliet: V, iii: "..But he which bore my letter, Friar John, Was stay'd by accident, and yesternight Return'd my letter back. "