Using smartphone at night can give you a 'hangover' Many people consider smartphones to be among the most important tools ever invented when it comes to increasing productivity of knowledge-based work, Mr Johnson said. Researchers asked 82 upper-level managers to complete multiple surveys every day for two weeks. They also surveyed 161 employees in a variety of occupations - from nursing to manufacturing and from accounting to dentistry. The surveys showed that night-time smartphone usage for business purposes cut into sleep and sapped workers' energy the next day. The study also compared smartphone usage to other electronic devices and found that smartphones had a larger negative effect than watching television and using laptop and tablet computers. Smartphones also emit "blue light," which is known to hinder melatonin, a chemical in the body that promotes sleep. "It can be a double-edged sword," Mr 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. The study will appear in the research journal Organizational Behavior and Human Decision Processes.