Sussing out sensitivity to light: How evening light impacts individuals’ circadian systems

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Science Translational Medicine  26 Jun 2019:
Vol. 11, Issue 498, eaax9568
DOI: 10.1126/scitranslmed.aax9568


Interindividual differences in sensitivity to evening light may present a new component of circadian disruption.

Before the invention of electric lighting, humans were exposed to either intense light (>300 lux) during the daytime or dim light (<30 lux) during the evening and night. Nowadays, humans spend the majority of their time in intermediate light intensities (30 to 300 lux), and exposure to artificial light at night is ubiquitous in modern societies. Light is the dominant synchronizer of our body clock and circadian rhythms. Specialized retinal cells sense the brightness of light and signal to the master clock in the brain, the suprachiasmatic nucleus. To assess the status of our body clock, investigators measure the onset of melatonin secretion under dim light (1 lux), which signals sleep onset. Suppression of melatonin by evening light provides a measure of an individual’s sensitivity to light. Previous studies determined that the effective dose for 50% suppression of melatonin (ED50) was approximately 100 lux. Although individual differences had been observed in a study of six individuals, and melatonin responses varied to a constant light stimulus, a systematic study of varying light exposures within a large group of individuals to fully assess interindividual responses was lacking.

Fifty-six healthy young individuals were recruited to a seven-week study by Phillips et al. The authors assessed the impact of weekly evening white light exposures in the range of 10 to 2000 lux on suppression of melatonin. As a group, the ED50 was 30 lux. Thus, typical indoor light (~50 lux) during the evenings will suppress melatonin in the majority of individuals and delay the onset of sleep. Remarkably, they observed greater than 50-fold differences in sensitivity to evening light across individuals. Some individuals had very high or very low sensitivity to light, in which it took 10 lux (dim reading light) or 400 lux (bright indoor light) to cause the same amount of melatonin suppression. Therefore, these individuals would respond very differently to the same home lighting environment in the evening. The participants in this study were all young and of Caucasian descent; future studies should assess whether this effect is consistent across other ethnicities and ages.

The notion that circadian rhythm disruption and sleep deprivation contribute to many conditions, particularly chronic disease, is gathering acceptance. This study identifies the large interindividual variation in response to light across intensities that we currently expose ourselves to in the evening. Individuals with very high sensitivity (low ED50 values) may be particularly at risk to circadian rhythm disruption and possible susceptibility to disease, especially in later life.

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