It starts at night. Every night, really. It’s time for bed, you’ve packed everything in to call it quits on the day, but you can’t fight that itch to pick up your phone or your tablet. Maybe you’re reading the day’s news. Maybe you’re watching Netflix. Heck, maybe you’re cramming in as much work as possible at the 25th hour. Or maybe you’re practicing the time-old technique of reading a book before going to sleep (on your handy tablet, of course). You know, because it helps you fall asleep. That’s why it’s been done for ages at bedtime.

Wrong.

You’re much better off avoiding that e-book altogether and trading it in for the old-fashioned printed version because the screen on that device of yours, no matter what you’re using it for, is killing the next day’s productivity.

You’ve probably already been told that staring at your smartphone, computer screen, and TV screen before bed tricks your brain into thinking it’s not the time for sleep. But it was a pair of Michigan State studies that found simply looking at our smartphone at night specifically kills the next day’s productivity more than anything else. “Smartphones are almost perfectly designed to disrupt sleep,” said Russell Johnson, MSU assistant professor of management. “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.”

For his first study, Johnson and fellow researchers had 82 different upper-level managers complete surveys over a two-week period. The second study reviewed the habits of twice as many people across a variety of occupations. In both studies, the surveys found that smartphone use specifically had a larger negative impact on quality of sleep and next-day productivity than the television or even laptops.

Smartphones are designed specifically to emit what’s known as ‘blue light’ so you can see them even in the middle of a bright day. We don’t look at our television screens outside or even our laptops without some kind of shade usually. So smartphones are unique in that they actually emit the most disruptive type of light possible. The brain stops producing melatonin and then it is officially harder to both fall asleep and even stay asleep through the night. According to Tech Insider, the disruption of sleep can impair your memory the next day as well as make it harder to learn and retain new information the next day. Once that cycle starts, blue light can even impact hormones that control hunger.

So what if you can’t put the smartphone down or step away from work until the last minute? The “night shift” mode on most smartphones is actually somewhat effective, believe it or not. Ditching the phone at night altogether is definitely preferable, but a nighttime mode on phone screens adjusts the colors of your screen to warmer tones. For phones that don’t have that mode built in, you can install mobile apps that will adjust display colors based on the time of day. But you still shouldn’t rely on this hack to be dependable. A Lighting Research Center study at Rensselaer Polytechnic Institute reviewed the results of young adults who viewed iPads between 11 pm and 1 am under experimental conditions. Some of them used nightshift while others viewed the screens at regular display settings. Subjects who used iPads without night shift had their melatonin levels decreased by 23% — nearly a quarter of your natural levels. Those who used nightshift and reduced the screen’s brightness had their melatonin reduced by 12%. So yeah, night mode helps, but even just a 12% drop in melatonin levels can be a sleep disruptor that kills productivity the next day.