You may have heard that bright lighting in the evenings can disrupt your sleep. But it turns out some of us are more sensitive to this effect than others. A new study suggests people鈥檚 threshold for their body clock hormones being disturbed by late-night light can vary by more than fifty-fold.
鈥淔or some people, a dim reading light might as well be daylight, and for others it might as well be darkness,鈥� says Sean Cain at Monash University in Melbourne.
Our bodies experience many biochemical fluctuations over the day, with a key regulator being a hormone called melatonin. This naturally starts rising in the evening, which promotes sleepiness, but the surge can be delayed by artificial lights, especially the blue-enhanced illumination from phones, computers and TVs. Some believe we can improve our sleep patterns and health by lowering light exposure in the evening and boosting it during the day.
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Most previous studies into the effects of light levels on melatonin have studied groups of people, averaging out their reactions. But Cain鈥檚 team looked at the individual responses to evening light among 55 men and women.
The volunteers had to keep a strict sleep schedule for up to eight weeks based on their preferred bedtime. One night a week, they came into the lab and were exposed to a certain light level 鈥� from dim to bright, in a random order 鈥� starting four hours before bedtime, and their melatonin levels were measured.
People had markedly different responses. In the most sensitive person, a very dim light level of 6 lux, equivalent to a few candles, was enough to halve their melatonin levels 鈥� which previous work suggests would delay sleep onset by 30 to 60 minutes depending on the person.
For the least sensitive person the same response required 350 lux, equivalent to harsh fluorescent lights in a store, brighter than would be found at home. On average, the light level required for such disruption was also lower than suggested by previous work. But the researchers did not measure if people鈥檚 sleep actually was delayed in this study.
There is no easy test for light sensitivity, but when applying this at home, 鈥渋t鈥檚 best to err on the side of caution,鈥� says Cain. He has installed home lighting that automatically becomes low and less blue from dusk onwards. 鈥淚 keep my lights as dim as I can without bumping into furniture.鈥�
PNAS
