Smartphones are ‘almost perfectly designed to disrupt sleep', researchers have said.

A US team carried out two studies into the use of smartphones for work purposes outside of normal working hours. It found that people who used their phones for work purposes after 9pm at night were more tired the next day in work.

"Smartphones are almost perfectly designed to disrupt sleep. 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," commented Russell Johnson of Michigan State University.

For the first study, 82 managers completed multiple surveys every day for two weeks. For the second study, 161 employees in various jobs were surveyed.

Both studies revealed that people who used their smartphones at night for business purposes were more tired in work the next day. These people had less energy and were less engaged with their jobs.

When compared with other electronic devices, smartphone use was found to have a bigger negative effect than watching TV and using tablets and laptops.

The researchers noted that aside from keeping people mentally engaged at night when they should be winding down, smartphones also emit ‘blue light', which is known to hinder the body's production of melatonin - a chemical that promotes sleep.

"So it can be a double-edged sword. The nighttime use of smartphones appears to have both psychological and physiological effects on people's ability to sleep and on sleep's essential recovery functions," Mr Johnson suggested.

He acknowledged that while one solution is to turn off your phone at night, this is not always practical in today's working world.

"There may be times in which putting off work until the next day would have disastrous consequences and using your smartphone is well worth the negative effects on less important tasks the next day. But on many other nights, more sleep may be your best bet," he said.

Details of these findings are due to be published in the journal, Organizational Behavior and Human Decision Processes.