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.
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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.
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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.