Lack of sleep may be the reason you spend so much time on Facebook during the day, new research suggest.

The study authors tracked 76 university undergrads for seven days and found that too little sleep was linked to more frequent online activities such as browsing Facebook.

"When you get less sleep, you're more prone to distraction," lead researcher Gloria Mark, a professor of informatics at the University of California, Irvine, said in a university news release. "If you're being distracted, what do you do? You go to Facebook. It's lightweight, it's easy and you're tired."

"There have been lots of studies on how information technology affects sleep. We did the opposite: We looked at how sleep duration influences IT usage," Mark said.

The study found a direct link between long-term lack of sleep, a decline in mood and increased Facebook browsing. It also found that the less sleep people get, the more their attention shifts between different computer screens, suggesting they're more easily distracted.

But the researchers only found an association between amount of sleep and daytime online activities, without proving cause and effect.

Mark expects to present the findings at the ACM Computer-Human Interaction Conference in San Jose, Calif., in May. Research presented at meetings should be considered preliminary until published in a peer-reviewed journal.