I loved sorting through the solutions (and reading the related explanations) for Friday's riddle, so here's a new one for this weekend's Weekend Genius Challenge:

Be it days or nights
It measures our slumber
But with letters in order
It is THE only number

WHAT is it? And WHY?

As with many riddles, they may be multiple near-correct answers, but of course, we're only looking for one. Send us your guess by commenting to this post, and please remember that submissions are moderated, so your post will NOT show up right away. If you're the first to correctly match our answer, you'll win a T-shirt of your choice from the mental_floss store. And my undying admiration.

Good luck!