Zuckerberg says the box emits a very faint light between six and seven in the morning. That's when the couple's two daughters usually wake up, and when Priscilla likes to get up too.
The thinking is, with the box, she won't have to check her phone, and won't get distracted or stressed out by seeing the time.
"As an engineer, building a device to help my partner sleep better is one of the best ways I can think of to express my love and gratitude," Zuckerberg wrote in an Instagram post.
He says the box is working better than expected, and he's putting the idea out there in case another entrepreneur wants to build sleep boxes for more people.