Just two hours’ exposure to a device with a self-luminous backlit display—a Kindle, iPad or other tablet computer—can cause a suppression in melatonin, which in turn could lead to trouble falling asleep, according to a study from the Lighting Research Center at Rensselaer Polytechnic Institute. Melatonin, a hormone secreted by the pineal gland in the brain, helps regulate sleep cycles.

A research team tested the effects of self-luminous tablets on melatonin suppression. To simulate typical usage of the devices, 13 individuals used them to read, play games and watch movies. The results showed that two hours’ exposure to light from an electronic display caused a 22% decrease in melatonin production, which can affect the ability to fall asleep.

Technology developments have led to bigger and brighter televisions, computer screens and cellphones, according to one researcher on the study. In order to produce the light that keeps the screen lit, the device emits light at short wavelengths, potentially delaying the onset of melatonin or suppressing the hormone’s production in the evening. The result? Falling asleep later and possible disruptions in a night’s sleep.

An hour’s exposure to light from the tablet did not seem to have a significant effect on melatonin production. However, after a two-hour exposure, there was significant suppression.

The results could urge manufacturers to design more circadian-friendly devices that either increase or decrease circadian stimulation depending on the time of day, reducing the amount of light stimulation in the evening for a better night’s sleep, and increasing it in the morning to encourage alertness.

But until then, the researchers have some recommendations for using tablet computers: dim the devices at night as much as possible, and if using them before bedtime, limit the amount of time.