The brain doesn’t distinguish between blue light from the sun and blue light from sources like our cellphones and laptops, so blue light at bedtime can signal to the brain that it’s time to be awake.
“It essentially tricks the brain into thinking it’s daytime in the evening and suppresses the mechanisms that promote sleep,” says Phillip Yuhas, PhD, an assistant professor at the Ohio State University College of Optometry in Columbus.
Research from the past decade suggests that exposure to artificial blue light in the evening tends to make sleep worse.
For instance, a study published in 2014 found that people who read e-books before bedtime were more tired the next day compared with people who read traditional books.Those who looked at e-readers had shorter rapid eye movement (REM) sleep, a deeper sleep state when key brain regions consolidate memory.
Dr. Yuhas recognizes that e-books have come a long way since this study. Many more now offer “night shade modes” that research has shown may reduce short-wavelength light like blue light. While clinical trials need to replicate these findings, the evidence suggests that screen-dimming can relieve but not fully prevent sleep disruption. “I still believe that the best option for patients is no screens before bed, however,” advises Yuhas.
Several studies suggest that blue light disrupts the production of melatonin, a hormone the brain naturally produces to signal it’s time to sleep.
Melatonin levels naturally start to rise in our bodies about two hours before we fall asleep, and remain high while we’re sleeping. But exposure to light — especially blue light — during this time may interrupt the natural production of melatonin, making it harder to fall asleep and stay asleep. In one study, blue light suppressed melatonin secretion for twice as long as the immediately longer green wavelength of light.
A person’s age may influence how they react to blue light. For instance, some studies have found that younger people tend to be more sensitive to light changes than older adults.
It’s important to note that research on blue light and sleep often has limitations that make it difficult to generalize findings. Studies tend to be relatively small and cover only short periods of time.
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