Sans is a MONSTER and not a HUMAN, so he doesn't follow HUMAN biology like a HUMAN skeleton. Same with Gaster and Papyrus.  

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Staring up towards the ceiling, Sans exhaled, as he was currently in the process of worrying himself out of sleep. Frisk on the other hand, looked like she was sleeping quite peacefully, as she was cuddled up quite close to him, letting out soft breaths as she slept. She was very clearly tired from her time at work, as her job likely took a lot of brain power. He frowned, staring towards her face which held a pretty content expression, and he went to gingerly brush a bit of her fringe out of her face, only to cause her to stir, and she went to sleepily blink open her eyes in confusion. 

"o-oh, sorry... i didn't mean to wake ya." Sans weakly smiled, hearing her let out a small tired giggle at him being apologetic. Even when he was still somewhat mad at her, his love for her was of course so much stronger. Because the truth is, even when she was driving him absolutely batty, he was still crazily, and utterly in love with her; he would be a fool to just say that he didn't suddenly care about her anymore, even when she was getting on his nerves. 

"It's okay... What are you still doing up? I would have thought you would be eager to get some sleep, with it being Leo and Luna's play tomorrow." Frisk murmured, causing Sans to weakly smile, and it was likely made obvious by his worrisome expression alone that he was still thinking about the thought of Frisk missing it. "Sans, I promise I won't miss it." She said, going to sit up and stop cuddling him, when she saw him looking so concerned. Saying that, she reached for the light that sat on her side of the bed, and she switched the lamp on. With a sigh, Sans sat up also. 

"but ya said you're workin' early in the mornin'. are ya even gonna be able to make it back in time?" Sans wondered. He didn't want it to seem like he didn't have faith in her. In fact, the biggest two reasons he was fearful of Frisk missing their performance, was one - he knew how upset the children would be, and two... He didn't want to think that he was actually going to have to say: enough was enough. Did he want to break things off with Frisk? No. Of course he fucking didn't. But if it just wasn't working, if she just couldn't listen, if the Mafia seemed more important to her... Then what else was he supposed to fucking do?

"I will. Listen to me, Sans. I promise, I will. Do you really think that you just aren't important to me anymore?" Frisk sadly asked, causing the skeleton to glance elsewhere, avoiding her eye. He didn't know what to think, when she kept ignoring his words of concern when it came to her being a part of the Mafia. He thought that when she saw how shaken up he had been when she almost died, that maybe that would make it clear to her how much it damaged him to have her so involved in such a dangerous thing. But it didn't do anything. "You see this?" Frisk added, going to hold onto Sans' hand which had the ring on it. "I wouldn't want to marry you if that was the case." 

"i just... i wish ya would listen to me..." Sans croaked, staring down towards her hand holding onto his, until he finally looked back up towards her face. "i just feel like i'm losin' ya. and i just don't... i don't want to have to let go of ya, frisk." Sans admitted, feeling like he might break down. It was funny, sometimes it just felt like Frisk had become the more serious one, while he had become the more sensitive one. But he couldn't help being sensitive, when he had so much to care about now than he ever had before. When you didn't care, it was easy not to be an emotional person; when you had more to cling onto, it wasn't like that at all. 

"Again, it's not because I don't want to listen. I just didn't want so many people to lose their jobs. If I hadn't stood up, then who was going to?" Frisk replied, softly placing one of her hands to Sans' cheekbone as she continued staring into his sockets which at times found it hard not to glance away elsewhere. "I love you, Sans. I promise I do." She reassured him, before placing a kiss against his mouth, which he at first hesitated to return. But when she kept kissing him, he could only close his eyes and return the kiss, knowing his soul was still utterly drawn to her. 

"i love ya too..." Sans quietly answered, meaning that to the bottom of his very soul. Because, again, he did, even if he was still angry with her. In fact, he found it extremely difficult not to begin blushing a light shade of cyan when she had decided move away from his mouth, only to start softly kissing and nibbling at the side of his spine. It was difficult not to be embarrassed by her doing that, even more so when she had never even done that before. He was so used to littering her neck with kisses, that it felt weird to feel her currently doing the same action, but to him. "nnh..." 

Part of him did just want to close his eyes and sink into it, even if it wasn't what he was used to. For a minute, that's exactly what he had started doing, as he just allowed her to lovingly kiss at the side of his spine, causing him to let out tiny sounds every now and then, until he decided... "n-no... not tonight." He carefully pushed her off, seeing her look a little disappointed, as he knew she probably wanted to prove in some kind of way how much she still loved him. But... It just didn't feel like the right time right now. "ya can prove yourself to me by bein' at leo and luna's play tomorrow." Sans told her. Because that was what was more important to him right now. She smiled.

"Okay." 

When she said that word: okay, she clearly genuinely meant it. Sans could see, just by that look on her face, and the gaze within her eyes, she was being as honest as she could be. And just that was enough to make him smile at her, feeling like he wanted to believe her. So he would. He knew he might have seemed like an asshole for not believing in her, but it was more because he was worried about their kids being disappointed, more than anything. At this point, it was more about the feelings of Leo and Luna, than it was about him. 

Even when it was the next day, and Sans found himself waiting for the play to start, with a bunch of other excited parents all sitting around, when he glanced at the time, he just thought: Frisk is just late. If she was only late, he would be completely fine with that. He would sweep it under the rug, if it meant she made the effort to even turn up at all. He had been positive at first, wanting to keep a good mindset to stop himself stressing, the more time ticked on. But when the play was finally starting, and Frisk still wasn't there... He was in a state of disbelief. 

Still, he tried to keep on a happy face for his children, hoping they would at least be happy to see one of their parents sitting in the audience. Sans knew, the moment Frisk got home, she was going to get a fucking earful. And that was no joke. Despite his smile on the outside, inside, he was fuming. He wasn't sure he had ever been this livid with Frisk, not even when she had said she had wanted to be the boss of the Mafia. At that point, he had been pretty pissed, but right now... There were no words to describe how he was feeling.

The moment Leo came on, dressed up in his little costume to clearly show that he was supposed to be playing the character of Joseph in this whole Christmas themed play, Sans' smile widened, as he really was super proud of him, even if Leo looked a little nervous. His son spotted him in the group of parents, seeing him giving him the thumbs up, but then Sans couldn't help but feel his soul sink when he saw a strange look flicker across Leo's face. He had clearly noticed the absence of Frisk, and there it was... That look of disappointment. 

What made it a million times worse, (as if it couldn't get any worse than this), was that not only did Leo have that expression of dismay on his face, but he even began to cry, as he was frozen to the spot. Tears ran down the child's cheeks like glistening crystals, at least until Leo tried to cover his face with his hands, as if seeing the sight of those falling tears would make it any easier on the father who had to bear seeing his son so disappointed. If Sans had a heart, he knew it would have completely shattered into two, especially with other parents "awh"-ing, because they thought that all he had was just a little bit of stage fright. But that wasn't really the exact reason.

Frisk had let him down.