¿Qué es la diferencia entre solo y sola? When used as an adjective, "solo" is the masculine form and "sola" is the feminine form. A man would say: "Estoy solo (I am alone)," a woman would say: "Estoy sola." So we know that Duo's sentence was spoken by a woman. If it was spoken by a man, it would be: "Yo duermo solo y como solo." Ésa es la sola diferencia.
When used as an adverb (meaning only/just) it is always "sólo" (or more recently "solo"). Example: Sólo quiero lo mejor para ti. (I only want the best for you). For more info see: http://www.spanishdict.com/translate/solo