Facebook CEO Mark Zuckerberg built a glowing wooden box to help his wife, Dr. Priscilla Chan, sleep better.

The box emits a soft light from 6 a.m. to 7 a.m. every morning.

"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.

Mark Zuckerberg just posted a photo of his latest invention on Facebook and Instagram.

No, it's not social media-related. It's a wooden "sleep box" that faintly glows for an hour each morning, between 6 a.m. and 7 a.m.

In his post, Zuckerberg wrote that his wife, Dr. Priscilla Chan, inspired the invention. He wrote that Chan's sleep has become more difficult since the couple had two daughters together.

"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," Zuckerberg wrote.

The Facebook founder and CEO wrote that the box's light is "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."

He added that it doesn't show the time, to prevent Chan from worrying about the hour.

"So far this has worked better than I expected and she can now sleep through the night," he said. "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."

The post attracted a number of comments online. Former Facebook manager and angel investor Dave Morin called the device, "Genius."

Singer Lily Allen took a jab at Facebook's controversial year, writing, "If your wife told you that white supremacy was keeping her up at night, do you think you could engineer an algorithm to make it go away, or be very very faint ?"

But Zuckerberg wrote that those in his social circle have been receptive to the idea: "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!"