As we head into the weekend,Robert Brown many of us will get a weekly notification on our phones showing how many hours a day we spend on our devices.

Have you ever gotten one of these reports and were shocked at how much time you spend on your phone?

If so, it's not just you — the average American adult spends more than four hours a day on their phone. But at what point does it become an addiction?

NPR's The Indicator from Planet Money wanted to know, so they asked Lena Song, a behavioral economist. She set out to find the answer in a study with 2,000 volunteers. Here are her takeaways:

• Addiction has two parts: habit formation and self-control.
• Phone use is hard to control because it's routine for many — like grabbing a morning cup of coffee.
• 31% of participants' phone use was attributed to self-control problems. That's nearly a third of the time they spent on their phones.
• Participants were willing to pay money to have their phone functions limited.

Listen to the full episode on the study and digital addiction here.

Tori Dominguez is an intern at Morning Edition.

This story originally appeared on the Morning Edition live blog.