So she HAS overslept like this before. That's pretty worrying.
I mean... it's entirely possible that she accrues sleep debt like the rest of us without suffering from the negative effects of it, so her body works to 'catch up' on missed sleep if she remains awake for extended periods of time. That's because sleep is more for the mind to 'defrag' and run simulations to break down and better understand/prepare for various events that have/could happen than it is for the body -not that the body doesn't make good use of the time for healing and recovery, given it's basically 'low power' mode, and more resources can be devoted to it than while awake. It's actually why getting a good night's sleep helps far, far more, both for puzzle-solving and information retention, than overnight cram sessions or running through the problem at hand over and over. (Naturally, the best is a mix -getting the information or problem active in the mind, then resting to let the subconscious mind work through it all.) And that's all just stuff that may have carried over from when she was human, or applies to us in a non-magic world.
Then there's a possibility it's tied to how her personal immortality works, since it appears to be different from the elf's. Personally, I'm leaning to the possibility it may be connected to her use of magic. Every instance so far that she's overslept, she had used magic shortly before, even in the story she told the group just now, and the magic she has used held far more power or complexity than Vio's lockpicking.