New York, Jan 23 (IANS): Switching off your smartphone at night means better productivity at work the next day.

“Smartphones are almost perfectly designed to disrupt sleep,” said Russell Johnson, assistant professor of management at Michigan State University in the US.

“Because they keep us mentally engaged late into the evening, they make it hard to detach from work so we can relax and fall asleep,” he warned.

In a pair of studies surveying a broad spectrum of workers, his team found that people who monitored their smartphones for business purposes after 9 p.m. were more tired and were less engaged the following day on the job.

Many of us consider the devices to be among the most important tools ever invented when it comes to increasing productivity of knowledge-based work, said the research published in the journal Organisational Behaviour and Human Decision Processes.

For the first study, the researchers had 82 upper-level managers complete multiple surveys every day for two weeks.

The second study surveyed 161 employees daily in a variety of occupations - from nursing to manufacturing and from accounting to dentistry.

Across both studies, the surveys showed that night-time smartphone usage for business purposes cut into sleep and sapped workers’ energy the next day in the office.

In addition to keeping people mentally engaged at night, smartphones emit 'blue light' that appears to be the most disruptive of all colours of light.

Blue light is known to hinder melatonin, a chemical in the body that promotes sleep, Johnson said.

“The night-time use of smartphones appears to have both psychological and physiological effects on people’s ability to sleep and on sleep’s essential recovery functions,” he added.