I swear it was called "A Study of Alicorn Sleep Patterns" and had Twilight watching Celestia sleep while hiding in her closet. Then Celestia finds her and things get fun.

The author is aKaPinkiePie and it seems they've pulled the story down. I have a saved copy if you want it.

Found it on FiMFetch if you still need it. I'd post the full URL, but the URL tool says SFW only.

You'll have to click Advanced Options and check all of the Mature boxes for the search to work.

The Study of An Alicorn's Sleep Pattern by aKaPinkiePie
Complete
Who confesses: Twilight
CLOP. Twilight spies on sleeping Celestia, is found out, sex ensues.

EDIT: never mind, southpaw already FimFetch'd it.