Dr. Gregory M. Marcus, Senior Author at the University of California, San Francisco, said: "This is the first study to directly measure actual screen time in natural environments and compare it to sleep quality. We did not rely on participant self-report, but rather utilized a mobile app that ran in the background and could capture exact screen time duration."

The research has made use of the date from the 653 adults enrolled in the internet-based Health eHeart Study.

The smartphones of the participants were provided with a mobile application that is capable of recording the screen time, or the number of minutes in each hour that the smartphones screen was turned on. Over a 30-day window, the app continuously collected the data from the participants.

A number of participants had self-reported their respective bedtime and sleeping period, and their personal demographics and medical information, as well.

According to the report in the journal PLoS ONE, the total screen time of the smartphones averaged 38.4 hours per 30 days and an average screen time per hour was 3.7 minutes, equivalent to one hour and 29 minutes per day.

In an email with Reuters Health, Dr. Marcus said: "At first glance, that we on average spend about an hour and a half of our time interacting with our phone every day may not seem surprising. But in thinking about the fact that this is an hour and a half of uninterrupted time, that is a pretty significant part of our day."

Teenagers and younger people are projected to have longer screen time. According to the study, the screen time of smartphones varied depending on age and race of people but was alike for users that have the same socioeconomic background.

Michael Gradisar of Flinders University School of Psychology in Adelaide, Australia said: "This is a lot better than the usual surveys where people estimate how much they use their phone. I was surprised that people are using their phones for one to two hours per day. I thought it would be more than that."

Meanwhile, Gradisar told Reuters through an email that the television which also involves a screen but isn't interactive may not affect people's sleep in the same way as smartphones do.

 "More screen time right around participant-reported bedtime was particularly associated with longer sleep latency (or a longer time to fall asleep) and reduced sleep quality," Dr. Marcus furthered.