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

-------------------------------

Needless to say, it would be a lie if Sans said he slept well last night, or if he said he even slept that much at all. He just kept having random moments of waking up in the middle of the night, after having nightmares of seeing Frisk, lying in that bed. Or at other moments he would have bad dreams about what happened to her... He wasn't there but... He could imagine her screaming; the pain she felt. He at least couldn't exactly get stabbed, but her... She had flesh, and organs, which would have no doubt been likely damaged by her attacker that day. He just had to hope with all his might and soul that he hadn't hit anything vital, because otherwise... 

Letting out a huge outtake of breath, the monster slowly sat up in bed, only to proceed to rub his sleepy sockets, in the hope that it might somewhat wake him up. However, it didn't really do much whatsoever. He was exhausted. But that's what he got for getting hardly any sleep. What made it even worse was that he had to get up to wake Leo and Luna up as they had to go to kindergarten today. He mentally groaned, as he wished right now he could just stay in bed and do nothing. But it just wasn't that easy. 

Pulling himself out of bed, he decided to put on black baggy pants, with a plain grey short sleeved shirt that had a pun on it, which heavily contrasted with the actual mood he was in. He then just tiredly threw his dark blue hoodie over the top of that with a frown as he honestly felt miserable. When he decided to go to the bathroom and look at his face in the mirror, he could see dark circles already appearing under his sockets from his lack of sleep last night. It really did show just how he couldn't live without Frisk anymore, if anything ever did happen to her. If he were to lose her... Just the thought alone as he stared back at his reflection was enough to cause the mask to slip and for him to gradually break down again, causing him to hold his face in his hands as he shook and he tried to be as silent as possible as he cried. 

He just needed her to be okay. That's all he needed. He also wanted that fucker to pay, so damn badly. He wanted to hunt them down and make them regret ever being born. But then the time he would spend doing that would take away from the time he could spend sitting at Frisk's side, just to make sure she was getting better. As furious as he was, he just wanted to put Frisk's health first before his own vengeance. Again, it wasn't like the old days where it was just his own life he was looking out for anymore. It hadn't been that way for a long time.

okay, sans... He thought, as his hands went to grip onto the sink in front of him, and he went back to staring at his reflection in the mirror. get a hold of yourself. He was acting as if she had actually died, but she hadn't. She was still fighting to live, and he should have believed in her, rather than thinking the worst. He wished he could be like Frisk, more positive, less cynical... He wished he wasn't an over thinker. Sometimes he wondered how Frisk put up with him, and just thinking that somehow managed to bring a small smile to his face through his tears as he glanced at the ring on his left hand. 

Somehow, whenever he looked at that ring, knowing that Frisk gave it to him, it managed to overwhelm him with so much happiness, that it was just difficult not to smile. She actually wanted to get married to him, as he did to her. Perhaps that was another thing she was fighting so hard for, so she would still be there for their wedding day. He just had to believe in her that she would fight hard enough. He could do that, right? Because honestly, if she did die; if she gave up, he would never forgive her. 

All he had to do was just try to hold himself together and hope for the best. Of course for him, that was difficult, because as he had already established over a thousand times, he was a negative over thinker. Frisk was so much better at being positive than he ever was or would be. But he had to try his best to keep on a brave face for the kids; he didn't want to upset them. So, after cleaning his face up by splashing water on his face from the sink, he put on his best smile and went to wake the kids. 

Thankfully, Leo hadn't really been putting up much of a fuss now when being told he had to go to kindergarten. Sans didn't know how he was really doing on making friends; he was at least hoping the small boy was enjoying his time there more than he was before. He just didn't look forward to the days when he and his sister would have to start going to the bigger school, because if kids could be mean at just the ages of 5 or 6, then he didn't like to see the way they treated each other after that. 

It was the usual that happened, Sans dropped the kids off at their kindergarten, wishing them both a good day, but with Frisk having been gone for 24 hours, it wasn't difficult to notice her absence. The skeleton sighed as he watched the two leave, before going to take his leave himself. He would have some time to be able to see Frisk while the two were learning, and then he would have to come back to pick them up again. It was likely going to be a long, exhausting day, but he was willing to put in the extra effort for his fiance and kids. 

So, once again, he found himself sitting at Frisk's side, holding onto her hand like there was no tomorrow. He wondered if this was what she felt like, when she had to sit at his bedside, hoping for the best, hoping that he would be okay when he had gone and almost shattered his soul in half. Back then, he had been so stubborn and still snapped at her, but she was always so sweet towards him, he remembered that moment like it was yesterday. How he woke up to her hand on his skull, and her fussing over him.

Glancing over his shoulder, Sans couldn't help but notice the presence of two other individuals who had entered the room, and it appeared to be none other than the fish monster, being closely followed by her wife, the yellow dinosaur. The skeleton couldn't help but narrow his eyes a tiny bit, only because he knew that Undyne was probably the one who had spoke to Asgore to put Frisk on that mission to stop that murderer from getting away. He knew that they probably trusted Frisk to be able to do it, because she was clearly skilled and she could take care of herself, despite what happened. But it still made him so fucking angry. 

"H-H-Hey, Sans..." Alphys spoke up, definitely finding herself stuttering a little more when she saw the skeleton holding Frisk's hand, and the stare he was giving the both of them. It was a look that made it clear he was mad with them, but he was internalising all that rage that could just blow up in a single second. It was extremely unnerving, knowing that he might just snap at the both of them at any moment. Though, he would probably yell at Undyne more than he would at her. 

"I'm extremely sorry about what happened, Sans. After all, I do feel partly responsible." Undyne spoke up, glancing towards Frisk with a bit of a saddened look upon her face. It was easy to act tough and act like it wasn't really her fault that Frisk had gotten hurt, but she had been the one to ask a lot of Frisk, especially when she had been someone who had delivered a lot of help to the CIA, countless times. Frisk was one of those people that actually made her really glad that the Mafia and the CIA had some kind of partnership going, so that they could work together when they needed each other. Growling a little, Sans stood up, as he let go of Frisk's hand, only so he and Undyne could be more face to face. 

"you're sorry? well, sorry isn't goin' to fuckin' fix the holes in her stomach, is it? you're sorry?" He said, finding it absolutely ridiculous that she was apologising. She just stared back at him, not really being that unnerved by his built up fury. Though, she did clearly feel sympathy, even if she wasn't put on edge by Sans' anger. "WHY THE FUCK DID YA NOT ASK SOMEONE ELSE? WHY NOT ME, WHY NOT-- I DON'T KNOW, ASRIEL? WHY FRISK?!" Sans seethed, as the pupils had completely disappeared from his sockets, and for the millionth time, tears began to prick at his sockets. He exhaled. "...he could have killed her... she could even still possibly die, and i don't want to lose her... i don't want leo and luna to lose their mom, and it's because of ya..." He trailed off and shook his head, before slowly going to sit back down. 

"W-W-We couldn't h-h-have known t-this would happen, S-Sans... Y-Y-Y-Yes, there, um, w-was a possible chance that s-s-something would go wrong b-b-but... T-There's always a r-r-risk, with, um, these, um, t-things." Alphys stuttered, trying to stick up for her wife who hadn't decided to yell back at the skeleton, even if part of her had wanted to shout back. What would that have really solved? She said she was sorry, what more could she really do? What did he expect her to do? He just sat there, with no pupils, not until they finally came back, and they slowly trailed back up from the floor to Undyne's face. Once again, he sighed. 

"...i know. sorry. i'm just... i don't actually blame ya, at least, not as much as i blame the guy who actually hurt her..." Sans admitted with a frown. "...i'm just lookin' for ways to take my anger out... just like old times, heh." Sans sadly laughed. He honestly just felt so lost without Frisk, when she wasn't awake and having things to say. She was someone who helped keep him calm, who made him feel more alive than he had ever been. But when she wasn't here... 

It was just hard without her.