Hoping for a productive day at work tomorrow? Then be sure to switch your smartphone off at 9pm. A new study indicates that using a smartphone for work-related activities in the late evening can leave people tired the next morning, and have a significant impact on a person’s productivity on the job.

Smartphone use strongly correlated to poorer quality sleep, and lower engagement at work the next day, even when directly compared to the use of other screen technologies during the evening, such as television, laptops, and tablets.

"Despite the fact that smart phones were not used as much, the smartphone had at least double the effect the next morning," says Dr Russell Johnson, assistant professor of management at Michigan State University and one of the authors of the study.

On average, participants in the study spent only five to 10 minutes on their smartphones after 9pm, compared to 45 to 50 minutes watching TV, and 20 minutes on computers. But even that brief period of use was more detrimental to workers. "Only smart phones impacted a person's engagement at work. As smart phone use went up, you could clearly identify that engagement went down. That's the finding that surprised me most," he told The Irish Times this week.

The paper, to be published in the research journal Organizational Behavior and Human Decision Processes , is based on two studies undertaken by the group, the first of 82 middle- to high-level managers, who completed morning and evening surveys every day for two weeks. The second survey, of 161 employees engaged in a variety of occupations, also included their use of other screen technologies.

Why do the study? “There’s been a lot of speculation about this, and a lot of anecdotal evidence, but really no rigorous scientific investigation of these possibilities,” Johnson says. It also seems that being a Digital Native from a more youthful, digitally adept generation, confers no benefits. Johnson says there was no evidence that younger workers who had grown up with computers, email and the internet coped better.

It also didn’t matter where people featured in the company – whether they were high-level managers or in service work. Whatever their role, smartphones had a negative impact on work productivity the next day. “With these studies, you can look at whether gender affects the results, or age. We didn’t really find anything consistent,” he says.

The only slight moderating factor was how much job autonomy an individual had. People with more autonomy – who would be considered people with more self-control – seemed to be able to re-focus better than those who are in less autonomous jobs.

The studies were set up so that participants had their own smartphone usage compared over time. This would mitigate for potential influencing factors such as whether someone had had a bad night’s sleep the previous night, or had worked late on a given day, or had a particularly challenging work day.

“We wondered, does smartphone use vary more between people, or for one person over time,” says Johnson. “For all of those using smartphones, there was more fluctuation across the days for a single individual, then between people.”

Participants were questioned early in the morning about their device usage and the quality of their night’s sleep, and then again in the late afternoon, to assess how they had functioned at work that day, and how depleted they felt, says Johnson. Participants were only assessed on the basis of their usage of their smartphones for work purposes, not for casual leisure use in the evenings.

The researchers also looked at “reverse causality” – whether it was possible that it wasn’t smartphone use that cause people to feel more worn out the next day, but maybe that if they felt less productive at work, this spurred them to use their smartphone more in the evening.

They were able to rule out that this was the case, says Johnson.

The study authors hypothesise that two factors are at work in producing such results. The first is the growing evidence that the use of screen technologies at night is disruptive to sleep.

There are studies which have measured the effect of light on sleep, says Johnson.

“They found that even a very small light, directed for a short period of time onto somebody’s arm or leg – even while they are sound asleep – is enough to disrupt the quality of their sleep.”

Smartphones are a source of light, he notes. People often bring their smart phones into the bedroom and leave them on, where throughout the night the screen will be lit by notifications, even though the sound is turned off. “Smartphones are a lot more invasive than other devices. They’re always powered on, and people are being splashed by the display. Whereas, with other screen technologies, you either turn the device off or it will go to sleep and the screen will shut down.”

And how about those of us who wake during the night and have a quick look at our email? Surely that was more potentially exhausting than another types of nighttime smartphone use?

Surprisingly, it seems that it doesn’t matter whether you check the phone a couple of times before you go to bed, or in the middle of the night-it all has the same negative impact.

“One thing we were able to look at were the times when people used their smartphones, and we didn’t find any discernible difference in the impact,” says Johnson.

The group also believe that people’s failure to disconnect from work completely in the evening, was the other major factor in their findings, he says.

“There is also a psychological impact. Using your smartphone in this way prevents you from disengaging.”

Studies have shown that people who engage in other activities, participate in a hobby, or otherwise disconnect from work once they clock off, are more productive the next day, he says.

Some German companies recognise this and have moved to ease workers off constantly accessing work email. Volkswagen and Henkel stop workers sending work-related email between Christmas and New Year’s Day. VW reportedly shuts off email servers from a half hour after closing until a half hour before work starts over the holiday period.

Prof Ian Robertson, of Trinity College Dublin's School of Psychology and Institute of Neuroscience, notes that the association between the "blue light" emission from screens and poor sleep still "remains speculative". Perhaps general stress levels lead to both greater smartphone use and poor sleep, he adds.

He also notes: “Generally working late on anything may cause sleep disruption because you are not relaxing into the night. It may be that the smartphone use is symptomatic of working late and that the laptop and tablet use is more associated with relaxing activities such as reading or watching movies.”

Over half the people in the United States, the UK, and Ireland now own smartphones. Many businesses issue them to staff, believing them to be a useful productivity tool.

Johnson’s advice? “There’s one obvious take-home from this research. Keep the smartphone out of your bedroom.”