Your more tech-savvy friends may be tracking your Facebook activity and making some strange inferences. An anonymous blogger has discovered that it is possible to record when your Facebook friends are online, offline, or idle, and whether they’re using Facebook on their computers or phones. They have been tracking some of their friends for a while now and has found it’s pretty easy to tell when people are sleeping and when they’re awake.

The owner of the Literally Words tumblr discovered that Facebook makes the information available to anyone, even if it is buried deep in the site’s backend. It’s not easy to develop on your own, but now that he or she have posted the code to run the operation for free online, it could become more common.

The creepy program takes advantage of the fact that Facebook tells you in Messenger which of your friends are online and how recently they went offline. When you dig deep enough, you can find the source code that passes this information to the graphic interface. Then all you have to do is keep your computer running the program and soon you’ll have detailed information on your friends’ Facebook habits.

The graph shows the Facebook activity of one of the developer's friends. 

The good news is that there aren’t a ton of very invasive uses for the information, but it is a little weird that all your friend technically have access to it. The blogger was able to create graphs of friends’ activity, indicating their usage time that do seem to resemble when the person finally logged off before going to sleep.

The blogger also suggested aggregating the information of all your friends to spot outliers — those people who have weird sleep habits and never seem to shut off Facebook for the night. So if you’re on Facebook instead of getting a good night’s rest, you may now be particularly vulnerable to exposure.

Of course, there are plenty of reasons why the data may not be perfect. Some people may keep their Facebook messenger idling at all times, and others may not use the feature at all, remaining permanently logged off. The blogger also points out that if someone is running the program themselves, they will appear to always be online.

“Look I’m not really sure why but I think I made a thing that makes graphs of when people are online on Facebook,” he or she writes in the very entertaining post (go read the full thing. “It sounds kinda creepy and uh it is. Read along so you, too, can be the NSA.”

So maybe it’s worth heading over to Github, downloading the program, and running it against all your friends. If you find someone who never seems to log off Facebook, then you can be pretty sure they’re running the same program as you. You could call them out, but then, of course, you’ll have to explain how you discovered them in the first place.

Photos via Literally Words, Getty