Translation:You wake up Marcus before the first hour.
Because he is the one who is still sleeping.
Otherwise the accusative case shows that he is the one excitatis is happening to.
Marcum is the accusative form of Marcus. The accusative form generally denotes (at least in the active voice) who is 'affected' by the action of the verb.
A good thing to remember however: esse (sum, es, est, etc.) does not use the accusative.
Because esse is a special kind of verb, like to be in English: a linking verb, which does not have an object but a predicative.
"Before the first hour you all wake up Marcus" is accepted. "Before the first hour you all wake Marcus up" is not accepted.