If you’re a devoted Facebook user, it’s alarmingly easy to tell when you sleep. A Danish software developer released a new tool that uses Facebook data to shed light on the sleep cycle of friends. Anyone using Facebook when they wake and immediately before bed could be tracked by their friends. The service is not as effective at tracking less regular users of Facebook.

“My point was not to spy on my friends,” said Soren Louv-Jansen, who lives in Copenhagen. “I want people to be aware that they’re leaving some digital footsteps everywhere they go.”

Louv-Jansen was motivated by his girlfriend’s frustration that her Facebook friends could see when she was last active on the platform. He started brainstorming and realized much more could be gleaned from that Facebook data.

Facebook stores timestamps on when its users were last active on Facebook and its Messenger service. Louv-Jansen’s tool automatically collects this information to create visual depictions of his friends’ sleeping habits. He said it worked extremely well on 30 percent of his friends, and somewhat effectively on the remaining 70 percent. He learned that sleep patterns are consistent Monday through Friday but random on the weekends.

Anyone with minimal programming experience can download and use his tool.

He worked occasionally on the project for five or six months before releasing the tool in December. Hardly anyone noticed his work until he published a related article on Medium over the weekend. Louv-Jansen said that 1,000 people have now downloaded his creation and almost 200,000 read his story.

“People should be aware whatever they do, they’re not alone, someone is always watching,” Louv-Jansen said. “I don’t want to say that Facebook is evil. This is just a side effect of what they’re doing.”

The Dane said he heard from Facebook on Tuesday that use of his program violated the social network’s terms of service. He said he was asked to discourage others from using it. Louv-Jansen said he has already stopped using the tool himself. But he refuses to remove it from Github, where he published it.

He’s a believer that the best way to raise awareness of the privacy at stake is to make the tool actually available to the public, rather than say it’s possible to track sleeping habits. He includes a disclaimer that the tool is for educational purposes only.

“I’m not proud of people starting to spy on their friends,” he said. “But maybe this can make everybody more aware of the consequences of our actions.”