We all already know it, but now it’s confirmed: nighttime smartphone use will ruin you the next day, reducing your productivity and focus after you wake up. Michigan State University business researcher Russell Johnson and his team produced a survey of American workers, asking how often they checked their phones after 9pm and how they felt at work. In short, they found correlation between late night smartphone use and performance.

The survey assessed “mental depletion/fatigue using a validated survey that participants completed the following morning” and noted focus and attention span.

“Smartphones are almost perfectly designed to disrupt sleep,” said Johnson. “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.”

Johnson asked managers as well as a broad sample of U.S. workers about their late night habits. It seems that using a smartphone can “zap” your energy more than watching TV or sitting in front of a computer. Because they are so close to your bedside and because they emit unrestful “blue light,” they are the worst culprits among everything keeping us from a good night’s sleep.

The associate professor concedes that we can’t always put down the phone, especially when our jobs are on the line.

“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,” he said. “But on many other nights, more sleep may be your best bet.”

“An obvious recommendation is to set some boundaries regarding smartphone use, such as completely powering it down after a certain time (e.g., 10pm). Managers can also help in this regard by not sending emails late in the evening,” he said.