University of Washington researchers have added yet another disastrous long-lasting effect of using smartphones. Researchers said that using mobile phones late at night seriously disrupts sleep patterns and weakens performance at work the following day. They have advised employees to switch off their devices by 9:00 p.m. for a fresh mind the next day.

"Smartphones are enormously valuable for helping people to fit work activity into times and places outside of the office," Christopher Barnes, an assistant professor of management at the Foster School of Business and lead researcher, said in a statement. "However, our new research indicates the greater connectivity comes at a cost: using a smartphone to cram more work into a given evening results in less work done the next day."

Barnes said that smartphones hinder a good night's sleep that in turn challenges an employee's efficiency the next day. As employees tend to work beyond office hours, they find it difficult to psychologically disengage from the pressing issues of the day.

For the study, the researchers conducted two experiments. The first experiment comprised of 82 mid- to high-level managers who undertook various surveys per day for two consecutive weeks. In the second experiment, 161 employees completed the same surveys. Plus time spent around television, laptops and tablets in the night were also monitored.

"Consistent with our hypotheses, we found that late-night smartphone usage cut into sleep and made people tired in the morning, and that, as a result, they were less engaged at work the next day," the researchers said.

The second experiment revealed that the harmful effects of television, laptop and tablets are similar to that of smartphones. But the effect was more prominent with smartphones.

Researchers said that team leaders should introduce a time table informing employees when exactly they should be online. Plus, the bosses should make sure to send e-mails in the morning rather than in the night

"The more important the job, the more important it is to work with a fresh brain," the researchers conclude.

The finding will be published in the journal Organizational Behavior and Human Decision Processes later this year.