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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He wanted nothing more than to be able to stay at her side all night, just watching her; protecting her. However, he knew that there was a big issue with that, as much as he wanted to do it. Back in the past, it was easy to just sit at each other's side and stay up all night, and not go anywhere. Now, they weren't just living for themselves anymore, but their kids too. Sans knew he couldn't just drop Leo and Luna on Papyrus all night without even a little bit of an explanation. 

Admittedly, he was fearful of Leo and Luna questioning where their mother was, because it was weird for her not to be home by now. He couldn't just tell them the truth, he couldn't tell two 6 year olds what had actually happened to their mommy. Just seeing their curious and confused faces as he was driving back home was enough to stress him out as he was trying to think of some convincing lie other than the fact that Frisk was lying in a bed all bandaged up and unconscious. 

She had to be okay. She would be. But then he couldn't help but think to himself: what would he actually do if she... Died? What would he even tell the twins that they wouldn't be able to see their mom anymore if that happened? He knew it was probably silly to think of such things, that he should just think she was going to be fine and that there was nothing to worry about. That was the thing, it sucked for him of all people to be put in this situation. Him, the over thinker. He wished he could say such a trait had left him a long time ago, but that would be a lie. He could still be an anxious mess if something in his life went wrong, and his mind wouldn't even stop for a second to think about it a million times over. 

What if something happened to her when he wasn't there? That was his biggest fear. He didn't want anything to happen, but if it were to happen, he wouldn't want to be somewhere else and not be with her. Or, what if she woke up when he wasn't there and she felt scared and alone? What if... What if... God damn it, his mind just needed to shut its fucking trap, as he pulled up to the house with a huge sigh. He really wasn't looking forward to lying to Leo and Luna, but it wasn't like he had much of a choice. All he had to do, was put on his best smile and come up with some kind of excuse. Besides, he had always been a good liar. At least, he had been 90% of the time. 

Climbing out of the car, Sans shut the door after himself, and went to walk up to the front door of the house. It was getting dark now, as it was coming up to night time. Taking his keys out of his pocket, he unlocked the door, before entering his home and closing the door behind him. He could see the living room light was still on, and by the looks of it the kids were still up. They seemed to be laughing and giggling, and when entering the room, he saw that Leo and Luna were in their pyjamas still seeming to play board games with their uncle Papyrus. A bittersweet smile fell across Sans' mouth, as he loved seeing the way his kids interacted with his brother. Truthfully, Sans thought that Papyrus would be amazing if he had children of his own one day, because there was just no way any kid could dislike that skeleton's charismatic personality. 

"Daddy! You're home! You missed me winning as Snakes and Ladders!" Luna squealed from where she was sitting, earning a small laugh out of Sans.

"sounds like ya have been havin' fun." Sans commented with a grin, while Papyrus nodded in agreement. "sorry to keep ya busy all day, bro. i didn't know i'd be gone all day." Sans shrugged, while Papyrus nodded in understanding. He really didn't mind looking after the kids for long periods of time, because he loved his niece and nephew. Any time he got to spend with them he really did enjoy. 

"IT'S FINE, SANS. THOUGH, CAN I TALK TO YOU FOR A MOMENT?" Papyrus asked, only for Sans to raise his eyebrows in confusion. Papyrus went to get up off of the ground, leaving the brother and sister in the company of one another as the two skeletons went towards the kitchen, which seemed to just be the place where all private conversations were held these days. The younger brother's smile faltered, and he exhaled. "I HEARD ABOUT WHAT HAPPENED TO FRISK. CHARA CALLED ME." He said, instantly seeing Sans' happy expression crumble, and he glanced away with those saddened, tired sockets of his. "HOW ARE YOU DOING?"

"...fine." Sans lied, even though that couldn't be any farther from the truth. He was anything but fine. True, it wasn't like Frisk was dead, that wasn't what happened. But... He just couldn't stop thinking, what if she did die? What was he honestly supposed to do then? Would he even be able to cope? He didn't think he would be able to. He was fine with looking after the kids by himself, knowing Frisk would come home after work, but knowing that she wouldn't be coming home for probably a little while tore him apart. If she had died, and it had been like that forever, where he would never see her again, he would utterly fall to pieces. 

"IT'S OKAY TO NOT BE FINE, YOU KNOW?" Papyrus replied.

"i told ya, i'm fine. she'll be fine. she has to be." Sans grumbled, already becoming a tad bit hostile, and that was made obvious by the frustrated tone of his voice. Papyrus couldn't blame him, he was very clearly upset with the current situation, as was he. Of course, no one could possibly feel as hurt as Sans did - the skeleton who loved and adored her with all of his soul. He would likely do anything for her. Sans didn't want to think of the possibility of her getting taken away from him forever. Papyrus sighed, and simply nodded. He didn't want for Sans to think that he had to pretend being okay when he wasn't, but at the same time, he was clearly emotional right now and Papyrus didn't want to stress him out. He had already been through enough for one day. 

"WELL... JUST KNOW IF YOU NEED ANYONE TO TALK TO, WE'RE ALL HERE. I'M SURE SHE WILL BE OKAY." Papyrus reassured him, and Sans only quietly nodded, not having much else to say. That being said, the younger brother thought it was probably best he got home now, as much as he wanted to be there for his older brother. Sans clearly was tired and didn't seem to want to be bothered, and he didn't want to push his luck and cause him to snap at him, (as Sans did tend to get a bit snappy under stress). So, he thought for now it was probably best to leave it alone. 

"Bye bye, Papywus!" Luna was saying, when the four were at the front door, and the two kids were giving their hugs and words of goodbye to their uncle. Sans was quiet during the entire exchange between Papyrus and the two little ones, and only said a small simple "bye" when he was out of the door. When closing the door, Luna pouted. "You seem sad today, daddy." She pointed out, only for Sans to put on his best smile.

"no, i'm okay. just tired, is all." He told her, only for Leo to not look that convinced. For the time  being, the son stayed completely silent, as their dad mentioned how they should get into bed because it was bed time. Luna happily agreed, while Leo just trudged up the stairs after his sister and Sans. Even for only being little, he couldn't help but notice the elephant in the room, and that was the fact that their mom wasn't here. She hadn't been home all day, and even the 6 year old felt that that was weird for her. 

"alright, sweet dreams ya too." Sans kissed each of his children on the head after having tucked them into bed. Luna smiled and nodded, while Leo didn't say a word. At least, not until his dad was about to leave the room, and he decided to call out to him, stopping him from leaving the room. Sans paused, before turning back towards him with that same smile. "what is it, son?" 

"...Where is mommy?" Leo pondered. 

The question that Sans was afraid of. It took everything in him to not allow his smile to falter, as he stepped back into the room and stood in the gap between Leo and Luna's beds. He knelt down, glancing both to his daughter and to his son who both now had their attention focused on him. "she's stayin' with aunt chara for a bit. nothin' serious, she just wanted to spend some time with her sister." Sans told the both of them, hoping that they would buy the lie that he had told. Luna seemed to buy it, saying that she hoped that Frisk had fun, while Leo on the other hand was momentarily quiet, until he seemed to accept what Sans had said. "just get some sleep." He lightly told them both once again, as he wished them goodnight for the second time, and finally went to leave the room.

When he was alone in his and Frisk's room, it was then that he finally began to break down all over again. He never really used to shed a single tear before knowing Frisk, and now he was letting out tears like a rushing waterfall, finding it difficult to stop them from flowing. He hated when Frisk wasn't here, when she wasn't laying next to him in their cosy bed, where they would often share cuddles with one another while they slept. It was odd and uncomfortable for her not to be here. He wasn't used to sleeping alone anymore, not when he was so used to her being at his side. She had hardly been gone for a day, as she was healing from her terrible injuries and he just knew that he missed her, as he was clinging to his pillow and sobbing into it.

Sans missed her... So fucking much.