The "you" is implied in the word "duermes." This rule goes for many verbs in Spanish, as well as other Latin-based languages.
I don't understand why this cannot just be "sleep" with the you implied as it is in spanish?
The you is not implied in Spanish, it is part of the conjugated verb. ;) "duermes" means "you sleep." Conjugated verbs have subjects "built in" that must be part of the translation into English: I, you, he, she, it, us, they, etc.
I'm curious, then, as to how you would issue a command? In English the subject is implied when you say "Sleep" but it also then acts as a command. This is also true in German, where you use the "you-form" of the verb but do not include a subject--it becomes a command. If command-forms of verbs are the same in Spanish, then "Sleep" should also be an appropriate translation here.
There are command forms in Spanish, but not structured like English of German. "you, sleep!" = duerme "you, don't sleep" = no duermas and others
for commands, if the verb ends in a, switch to e and if it ends in e switch to a. now you have imperative