Ask a Neuroscientist

Q: I read that I should reduce my screen time in the evenings to help me sleep better by reducing my exposure to blue light. Why is this? Why is blue light specifically bad?


It seems that bright light (screens even at their dimmest are quite bright!) for a few hours before bedtime can affect the quality of human sleep. To understand why this is the case, let’s step back and see how sleep is regulated. Human sleep cycles are determined by an underlying biological clock--a pattern of biochemical feedback that repeats itself approximately every 24 hours. This pattern is called a circadian rhythm, from the Latin “circa diem” meaning about one day. These circadian rhythms are controlled by the area of the brain called the suprachiasmatic nucleus (SCN) and, being an internal phenomenon, they do not rely on light to happen (humans still have a circadian rhythm when kept in complete darkness). However, light helps adjust this rhythm to the light/dark cycle each day. Upon sensing light, the pineal gland in the human brain decreases the production of melatonin, a sleep-inducing hormone that plays a crucial role in helping set the precise time of our circadian rhythm. For millennia as the sun set and the landscape darkened, the pineal gland’s melatonin production would slowly rise. In the present day, we are often exposed to light well into the evening past when the sun has set. As you might have guessed, this exposure to light delays the onset of melatonin production. But why all the buzz about blue light? Some scientific studies have indicated high energy light (such as that in the blue area of the spectrum) best suppresses melatonin production. This may be because blue light sensitive cells in your eye communicate information directly to that central regulator of circadian rhythms, the suprachiasmatic nucleus (SCN), which passes along the message to the pineal gland to not start melatonin production just yet.




-R. Keating Godfrey, PhD

She/her

Research Scientist at the University of Arizona

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