It’s been known for a while now that exposure to short-wavelength “blue light” from our phone and tablet screens can harm our sleep. Apple introduced a feature called “Night Shift” in the latest operating system for the iPhone and iPad that cuts down the blue light in the evenings before bed. A popular add-on called F.lux does the same for desktop computers.

But researchers were not clear just how bad blue light can be for our sleep until a new study conducted by the University of Haifa and Assuta Sleep and Fatigue Institute was published in the journal Chronobiology International.

Led by Prof. Abraham Haim from the University of Haifa, the researchers sat 19 healthy subjects aged 20 to 29 in front of computer screens between 9pm and 11pm in the Assuta sleep lab. The participants were exposed to four types of light: high-intensity blue light, low-intensity blue light, high-intensity red light and low-intensity red light.

Following the exposure, they were connected to instruments that measure brain waves and to a wearable device called “an actigraph” that monitors when a person is awake and when the person is sleeping based on movement. Participants also filled in a sleep diary.

The researchers’ conclusion: exposure to blue light reduced the duration of sleep by approximately 16 minutes on average. This occurred in large part because exposure to blue light suppresses the production of melatonin, a hormone critical for sleep health. Exposure to red light does not affect melatonin production.

Melatonin regulates the body’s biological clock so that “when the body moves into sleep it begins to reduce its temperature, reaching the lowest point at around 4:00 am,” Haim explained. “When the body returns to its normal temperature, we wake up.”

The blue light led the body to maintain its normal temperature throughout the night, rather than cooling down and warming up.

The most significant finding was that exposure to blue light drastically disrupts the continuity of sleep. Whereas after exposure to red light — at both intensities — people woke up an average of 4.5 times (including unnoticed awakenings), following exposure to weak blue light, 6.7 awakenings were recorded. And with exposure to strong blue light, that rose to as many as 7.6 awakenings.

As a result, participants reported in their questionnaires that they felt more tired and in a worse mood after blue light exposure.

The study didn’t look at the effect of exposure to blue light during the day, but Haim notes that exposure to screens “is an integral part of our technologically advanced world and will only become more intense in the future. Fortunately, various applications are available that filter the problematic blue light,” reducing the damage our devices are wreaking on our sleep.

And the good news is that it’s “not the screens themselves that damage our biological clock, and therefore our sleep, but the short-wave blue light they emit.”