Facebook CEO and billionaire Mark Zuckerberg has built a sleep box for his wife Priscilla so she could sleep well at night.

Priscilla, according to Mark, had been having difficulty having good sleep because of the kids. The sleep box was constructed in such a way that it emit some light when it;s the right time for her to wake from sleep.

He made the revelation via his Instagram page:

Being a mom is hard, and since we’ve had kids Priscilla has had a hard time sleeping through the night. She’ll wake up and check the time on her phone to see if the kids might wake up soon, but then knowing the time stresses her out and she can’t fall back asleep. So I worked on building her what I call the “sleep box”.

It sits on her nightstand, and between the hours of 6-7am it emits a very faint light — visible enough that if she sees it she’ll know it’s an okay time for one of us to get the kids, but faint enough that the light won’t wake her up if she’s still sleeping.

And since it doesn’t show the time, if she wakes up in the middle of the night, she knows to just go back to sleep without having to worry about what time it is. So far this has worked better than I expected and she can now sleep through the night.

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. A bunch of my friends have told me they’d want something like this, so I’m putting this out there in case another entrepreneur wants to run with this and build sleep boxes for more people!