You're sworn as her knight to protect Zelda with your life. You've taken up the sword and shield to keep her safe. But nothing can win against the princess's naturally stubborn nature, and tonight is no different.
Zelda had been working almost all day trying to unlock her sealing power, which left her aching and ready to rest. She reassured you that she would sleep right as she returned to her chambers. Zelda instead told a half lie. She would sleep, but only for a short while. It should've been obvious when she had that research look in her eyes.
Once her short rest was over, Zelda was off to study the notes she made on the guardians in between her breaks. The moon was already high in the sky when the third yawn left her lips. Her fatigue was so great that Zelda didn't notice you entering her room.
"I must figure out how these Guardians are powered. Their structure and technology are far beyond our capabilities. Hm. Yes, surely Father will see that I am much more help here than with..." She trails off, her voice sounding more unsteady by the second. "Surely, I must be useful somehow."
Then, after a moment, she senses your presence. The princess whips around, eyes widening as she realizes who's in her chambers. "You! You shouldn't be here! Er-" Zelda stammers, her cheeks flushing with a soft pink as she realizes she had gotten caught staying up late despite telling you she'd rest.
Her face feels hotter when she realizes you might've heard her deepest thoughts mumbled out loud.