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Late sunlight buffers night screens
Someone on a computer at nine or ten at night gets less damage to melatonin, blood glucose, and sleep if bright light reached their eyes in the later part of the day. Clock times don't matter, since everyone wakes and sleeps at different times. What counts is sunlight in the eyes without sunglasses, from a window, a balcony, or a quick walk around the building.
Adolescents are the exception. Late daylight doesn't buffer them from screen light at night. Their cortisol runs slightly but significantly higher in the evening and at night, and the next morning it struggles to get up.
Falling asleep fine after a screen doesn't rule out the damage, since the blood glucose effects and the next-morning cortisol effects are where it shows.
Why do teenagers' body clocks run later?
During puberty, the circadian clock shifts later, so melatonin starts rising an hour or two later than in childhood. Mary Carskadon's research at Brown University documented the shift in adolescents. A similar delay appears in other mammals, including rats and monkeys. That points to a biological cause as well as a social one.
How does earlier light change evening effects?
In a 2002 study in the Journal of Clinical Endocrinology and Metabolism, Michael Hébert and colleagues found that people who spent time in bright light earlier in the day lost less melatonin to evening light than those kept in dim rooms. Researchers call this the effect of prior light history. Retinal cells appear to adjust their sensitivity based on what they have recently seen.
What did a study show about sleeping with light?
In a 2022 study in PNAS, Phyllis Zee's group at Northwestern University had young adults sleep one night with a dim room and another with overhead room light. After the lit night, their hearts beat faster during sleep, and their insulin resistance was higher the next morning. The volunteers reported sleeping about as long. The lights were about 100 lux, as bright as a typical living room.
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