I haven’t actually thought this through much but here we go! I am assuming you mean while Marius was “““sleeping on Courfeyrac’s extra mattress”””:
They rarely begin their day at the same time. Marius’s sleep is fitful and he generally wakes up early regardless of when he fell asleep, where as Courfeyrac, to paraphrase Gandalf, “awakes exactly when he means to,” which is often fashionably late. This is unfortunate given how much time it takes him to get ready to greet the world; if Marius hasn’t left the flat already by the time Courfeyrac gets up, he will have secondhand anxiety about Courfeyrac not finishing his morning ablutions on time and being late to whatever appointments he may have that day, and he tries fruitlessly to get Courfeyrac to forego immaculate looks. If Courfeyrac is not awake to stop him, Marius furtively prepares a rather sparing breakfast for himself before heading out the door. These scenarios, one or the other, are how most mornings go.
Then there are the rare mornings when Courfeyrac wakes up before Marius, and he stays in bed just to look at him and curl up against him (Courfeyrac may be shorter than Marius but there’s something about the boy that makes him feel compelled to be the big spoon). He can tell when Marius wakes up because there is always a soft noise of confusion and surprise, but once he figures out what the warmth and pressure against him is, Marius breathes a contented sigh and relaxes back into his friend. He hums happily when Courfeyrac begins to stroke his hair. Eventually, though, one of them sits up, kisses the other, and they both stretch themselves lazily as kittens and prepare to face the day. On days like this, Marius does not let himself worry that Courfeyrac treats him to a nice breakfast at their favorite nearby restaurant.
[I didn’t even make it to a full Day In the Life, but hopefully some off-the-cuff waking up headcanons will suffice?]