After midnight, Sen. Chris Murphy was clearly getting tired. He'd been speaking since Wednesday morning, and his filibuster had rolled into early Thursday morning. Although he had the help of other senators asking "questions" (aka long, long speeches that eventually asked a question), the filibuster was Murphy's, and that meant it was he who had to follow the rules of a filibuster. But why? Why can't you sit down during a filibuster? It basically comes down to semantics.
Filibusters have been around for at least a century. And as you've probably seen from previous ones or from pop culture, there are some very, very specific rules a politician must follow. One of them includes that a person must stand for the entire time, never taking a seat (a là Leslie Knope on roller skates). The reasoning behind it is simple: once you sit down, you signal that you're done speaking.
As The Boston Globe reported in 2013, if a senator were to sit down, he or she then "yields" the floor, and the presiding officer can move to recognize someone else to speak. This effectively ends the filibuster, hence why Murphy has been standing for 14 hours as of around 1 a.m. ET. He can walk around the floor, he can stay at his podium, but he cannot take a seat, and he certainly cannot stop talking.
Luckily, other senators are also allowed to speak. Like Cory Booker, who gave an impassioned speech and some much needed energy to the floor, senators are allowed to ask questions, but they are allowed to take a good amount of time to do that, which likely helps Murphy out a lot when he's running out of things to think about. Murphy also isn't allowed to take bathroom breaks. He can't eat anything, though he is allowed to have water.
It's no easy task, which is undoubtedly why it doesn't happen very often. But to Murphy's credit, he's stayed on topic the entire time. He has named victims of mass shootings, he has given their stories, he has shared why he feels so passionately to make this stand (compare that to, say, Ted Cruz, who read Green Eggs and Ham in a filibuster attempt against Obamacare). Murphy never strayed from recognizing the victims and the importance of his filibuster, all while standing in what I'd guess aren't very comfortable shoes.