One of the perks of having an inattentive father and generally empty household was that no one really noticed if you weren't there in the middle of the night. So Adrien took full advantage of this situation, sneaking through the window with Plagg and vaulting his way through Paris, towards the ever so familiar bakery. Sometimes he'd forget to text Marinette that he was coming, and just show up late at night. Usually, though, he'd text her. He'd crawl through the windows, and the fun began.
Some nights, when Marinette was working on a big project, he would just sit there in silence and play on his phone while Marinette worked, just enjoying the other being there. Other nights, they'd watch a movie or play a game, speaking only in whispers unless her parents were out, not daring to get caught. Her parents just thought she was on the phone with someone if they heard her, so they wouldn't interrupt anyway. Some nights when they were exhausted from an Akuma fight earlier that day, they'd just lay down and chat for a while. But no matter what kind of day it was, after all their talking or playing or working was done, Marinette would set an alarm on her phone, a little earlier than her normal time, and the two would crawl into bed together. They'd be wrapped in each other's arms, legs tangled together and heads pressed close, and they'd fall asleep. Tikki and Plagg would follow, sleeping on a tiny kwami bed hand-made by Marinette. They never did anything intimate in bed, simply rested together and warmed each other up, enjoying being held by the one they loved.
The early alarm would go off, signaling for Adrien to head home, despite his constant arguing to stay. He would, eventually, climb through the window and go home, where nobody was the wiser. Marinette would take the time to slowly wake herself up, or sometimes doze back off. Either way, it was actually pretty efficient because it helped her wake up earlier, and not be so late to class.
And when she and Adrien saw each other at school, they'd give each other a big smile, knowing how much the nights meant to the other.