That’s not how sleep works, Nat. You don’t actually get to “bank” it.

I think this scene and the one right before it are the only times we ever get to see Nat not wearing some sort of uniform.

↓ Transcript
Panel 1: The next morning, the three girls say goodbye to Nat's dad as they head out the door. Nat: "Bye, Daddy." Nat's dad: "Good luck, Cupcake! Do your planet proud!"
Panel 2: Nat has changed out of her uniform except for her hat and her boots. Her braids are undone and she's wearing a short dress. She rolls her eyes. Nat: "Dad, I'm just a cadet, innit! They're not gonna deploy me to space. Not for years, yet. If ever."
Panel 3: Nat's dad, enthusiastically: "Maybe if you're lucky! There never was a Co'erence Crisis while I was on active duty. They say there'll be one in the next few years!"
Panel 4: The girls make it past the front gate to the sidewalk. Nat, to Maida and Zaynab: "Well. That's over wiff."
Panel 5: Nat: "And fanks to him being in love wiff Maida Martian, I 'ardly had to listen to any of his moping!"
Panel 6: Nat: "Listen, slags. I know you're 'eaded to the station straight away, but I've got anovver two days of leave."
Panel 7: Zaynab: "But you're not... spending it wiff your dad?" Nat: "Hell no! I did my time."
Panel 8: Nat: "I've banked enough sleep I should be able to stay up all night."