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Klodiana Lanaj sleeps with her smartphone. Worse, her habit has been to respond to e-mails that come in late at night.

As a management professor at the University of Florida, she has watched as companies increasingly purchase smartphones for employees, to allow more flexibility and quicker responses to work issues. After finding her own sleep disturbed and her energy flagging, she began to wonder whether the net result of companies buying smartphones for their staff was higher or lower productivity.

Teaming up with two other academics who sleep with their smartphones – Russell Johnson of Michigan State University and Christopher Barnes of the University of Washington – she now has what appears to be a definitive answer: Smartphone usage late at night disturbs sleep quality and reduces energy and work engagement the next day. If you find yourself distracted and unable to focus at work, it may trace back to your smartphone usage in bed.

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"Smartphones are perfectly designed to disturb sleep," she said in an interview. When notifications are allowed, the phone will light up as an incoming message arrives. But light can inhibit the production of melatonin, a crucial hormone that tells the body it's time to sleep. One study found that illumination well below typical indoor light can suppress melatonin, and another showed that even a simple light pulse shined on the back of the knee interrupts the human circadian rhythm.

She also looked at research that probed what is known as "sleep hygiene," those habits that ensure a good night's sleep and those that work against it, such as drinking coffee and using the bedroom for activities other than sleeping.

Poor sleep hygiene has been linked to lousy sleep – not as deep and as recharging as we seek. Moreover, smartphone use late at night keeps you from psychologically detaching yourself from work, perhaps because it triggers ruminations about the job, Prof. Lanaj figured.

Since sleep is meant to replenish energy, we may find our energy depleted the next day. Our work can seem subpar. We feel less creative. This depletion also affects our self-control, studies have shown. "If you're depleted, it's easier to give in to distraction and do things that are easier – you're more likely to look at Facebook or do e-mail rather than dig into work in the morning," she explained.

Her first study involved 82 managers enrolled in weekend MBA classes at a large university in the Midwest, unaware of the purpose of the study, who answered a series of questions twice a day that included a look at their smartphone habits and an evaluation of their energy for 10 consecutive working days.

The more the managers used their smartphone late at night, the less they slept. As well, the more the smartphone usage, the less energy they perceived themselves to have the next morning. Moreover, the researchers found that people who were listless and groggy in the morning tended to have less engagement with work through the remainder of the day.

Interestingly, the study found job control could offset some of the impact of late-night smartphone use on productivity. Those who could choose to devote themselves to administrative work or other routine duties after some late-night work fared better.

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If this is true about smartphones, what about late night use of computers, tablets and television? To find out, they conducted a second study for 10 working days of 161 people who were recruited from Mechanical Turk, a crowdsourcing site. It drew similar results to the first study with respect to smartphone use, and also raised concerns that late-night computer work showed a similar impact the next day. Television had a lesser effect, and tablets none at all, perhaps because their use is more for leisure.

The message for organizations, Prof. Lanaj feels, is to be wary. Giving employees smartphones may help them to stamp out fires from home. But it also is associated with loss of productivity, and the overall impact might well be negative.

For individuals, the lesson is to watch your late-night smartphone usage, shutting it off, keeping it in a room other than your bedroom, or turning off notifications.

She still sleeps with her smartphone – but the notification feature is now turned off.

Harvey Schachter is a Battersea, Ont.-based writer specializing in management issues. He writes Monday Morning Manager and management book reviews for the print edition of Report on Business and an online work-life column Balance. E-mail Harvey Schachter

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