Maybe if Chat Noir were here, he'd be able to tell from the tilt of her shoulders or from the tightening of the muscles around her eyes and mouth or from the set of her jaw that she was feeling hopelessly tired and was just barely containing the tears that threatened to spill over, but a different blonde was standing in front of her. One with green eyes just as beautiful and who she loved just as much, but not one who could read her body language like an open book.
Chat and her had learned to know exactly what even the smallest changes meant so that they didn't always need to communicate verbally during a fight. But Adrien was not Chat Noir and he didn't know that she hadn't slept at all last night due to an akuma attack and then having to study for a test, nor could he have any way of knowing that she hadn't slept at all for the past four nights.
There was no way, even though he no doubt noticed her lack of disastrous stuttering, that he could see that every ounce of her energy and concentration was being directed to helping her stay upright and coherent enough to carry on this conversation.
Whether or not she actually realized that she had collapsed onto her desk and was now murmuring only partially coherently was debatable. Whether or not she realized that she was talking to Adrien and not to the Chat Noir of her fatigue-induced delirium was also debatable.
Nevertheless, she proceeded to try to convince Chat that she was perfectly capable of taking this patrol, not forgetting the pet names, before she trailed off and her breathing deepened until she fell into a restless sleep.
Meanwhile, Adrien, who had tried to wake her when she had face-planted on her desk, was now sitting in shock at the discovery that Ladybug was Marinette. That is, until he remembered that Ladybug was currently having an impromptu nap on her desk which, he realized, was because she was definitely not getting enough sleep.
So, because Adrien was Chat Noir and because no one was in the classroom, them being the first people to arrive that morning, he transformed, jumped out a window, and carried Marinette back to her home while she slept the whole time.
When he got there, he put her in her bed, called in sick for both of them, and sat at her desk to watch over her and wait for her to wake up.
When it became obvious that she would not wake up for a while, he did some homework online, convinced his father and Nathalie to let him "sleep over at Nino's after my photoshoot" (he should get a medal for that), turned off her alarm, and talked to Tikki and Plagg. He only left twice. Once, in the afternoon, when her parents, who thought she must have come home from school without them noticing and fallen asleep immediately, came up to check on her, and again later in the evening, when he went to patrol the (thankfully akuma-free) city and to get food.
When she woke up the next morning (why hasn't my alarm gone off yet and why am I not in my pyjamas?), she found Adrien Agreste (what?) sleeping on her sofa with rumpled clothes and Tikki sleeping on a pillow (is that Chat Noir's kwami sleeping beside her?).
Understandably, she said, "I hope this is just a really, really, really wierd dream."
That woke up Adrien who jumped up and exclaimed, "My Lady! You're awake!"
Despite Marinette's best efforts, she had just woken up to one of the most improbable scenes, and it was taking her a few minutes to process everything. With a frown, she said slowly and hesitantly, "Chat?"
Adrien, who was starting to realize that it all might be too much for her to handle when she had just woken up, said sheepishly, "Hi."
Still confused, she answered, "Am I dreaming?"
He chuckled a bit nervously, and went on to explain everything that had happened. It took Marinette at least half an hour to piece together everything he was telling her but, she finally had enough presence of mind to ask for the time which turned out to be nine o'clock on Saturday morning. She realized her parents would be expecting her any minute to come help in the bakery, so she sent Adrien off through the window and got dressed.
She spent the rest of the morning in a slight state of shock, the familiar work keeping her mind off of everything. It wasn't until a break around noon that it hit her full-on and she spent approximately twenty minutes freaking out in her room with Tikki. But, because Marinette was Ladybug, smart and sensible, she managed to come to terms with her new reality, calmed down and went back to work.
That afternoon, she met up with Chat Noir for patrol. They spent a while talking and figuring out the new dynamics of their relationship (among other things) and then they parted.
Afterwards, because Alya had been texting Marinette asking where she was, why she wasn't in school and why she wasn't answering, Marinette told her a (very) abridged version of what happened Friday morning, and that her and Adrien were now dating.
Monday morning at school, when Alya gushed to her friends about the picture she got of Ladybug and Chat Noir kissing (with memories intact) instead of the usual fist bump after akuma fights, Adrien and Marinette just exchanged knowing smiles.