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Blue Light Affects Melatonin Secretion

Updated: Apr 23, 2020

Light is the most powerful cue for shifting the phase or resetting the time of the circadian clock. We know that melatonin is present at low levels during the day, begins to be released a few hours before bedtime, and peaks in the middle of the night. Studies have shown that light suppresses melatonin, such that light in the early evening causes a circadian delay (resets the clock to a later schedule), and light in the early morning causes a circadian advancement (resets the clock to an earlier schedule).

Light is made up of electromagnetic particles that travel in waves. These waves emit energy and range in length and strength. Blue light has a very short wavelength, and so produces a higher amount of energy than the other colors in the visible spectrum. Blue light is commonly emitted from screens (flat screen televisions, computers, laptops, smart phones, and tablets), electronic devices such as e-readers, and fluorescent and LED lighting.

In today’s age, after using a computer throughout the day, is it common for people to come home from work or school and relax by watching television, surfing the internet, or reading an e-book. In fact, data from a 2014 study by Milward Brown found that Americans spend an average of 7.5 hours per day looking at screens. However, health care providers have begun to caution their patients against using light-emitting devices before bed. Why? The light from our devices is “short-wavelength-enriched” meaning it has a higher concentration of blue light than natural light.

While light of any kind can suppress the secretion of melatonin, blue light affects melatonin levels more than any other wavelength. Harvard researchers and their colleagues conducted an experiment comparing the effects of 6.5 hours of exposure to blue light to exposure to green light of comparable brightness. The blue light suppressed melatonin for about twice as long as the green light and shifted circadian rhythms by twice as much (3 hours vs. 1.5 hours). For patients who report insomnia and/or fatigue as a significant symptom, examining melatonin levels and obtaining a detailed history about nighttime LED exposure may prove beneficial.

How can we reduce our blue-light exposure before bed? For those who just cannot turn off digital devices, here are a few tips. Dim the brightness of devices, or make use of programs that filter out short-wavelength light in the evening. Use dim red lights for night lights, as red light has the least power to shift circadian rhythm and suppress melatonin. Consider wearing blue-light blocking glasses. But the best, and least popular answer, is to simply turn off these devices at least 2-3 hours before going to sleep!


  • Schmerler, Jessica. “Q&A: Why is blue light before bedtime bad for sleep? Two neuroscientists discuss how blue light negatively affects health and sleep patterns” Scientific American. September 1, 2015.

  • Accessibility verified 4/19/16.

  • Woollaston, Voctoria. “How much time do YOU spend looking at a screen? Infographic reveals extent of tech addiction across the globe.” Daily Mail. May 29, 2014.

  • Accessibility verified 4/19/16.

  • Harvard Health Letter. “Blue light has a dark side.” Harvard Health Publications; Harvard Medical School. May 1, 2012; updated September 2, 2015. Accessibility verified 4/19/16.

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