By Maryna Mullerman ’20
Artificial light and technology have been known to affect human sleep patterns. Scientists think that the circadian clock — physiological 24-hour cycle — might shift in the future. Sarah L. Chellappa and researchers from Harvard Medical School aimed to understand how chronic exposure to light at night affects human health and behavior. They explored how differences in light sensitivity between sexes affect brightness perception, attention, cognitive brain function, and sleep.
The study involved 32 healthy participants — 16 men and 16 women — who were matched according to their ages, body mass indices, and ethnicities. The study’s experimental procedure started approximately 10 hours after each participant’s usual wake-up time. Individuals were exposed to 1.5 hours of dim light, 2 hours of darkness, 2 hours of bright light, and 45 minutes of dim light before their usual sleep times. Two light types were tested: 6500 K, blue-enriched light and 2500 K, non-blue enriched light. The researchers investigated light preferences by asking the participants to describe their comfort under specific light conditions. “Light sensitivity” was defined by brightness perception. Cognitive function was explored through auditory psychomotor vigilance task (PVT), which was associated with circadian rhythm and sleep requirement. PVT was performed at different light intensities, indicating attention span. Electrical activity of the brain was recorded during participants’ sleep periods, including those involving non-rapid eye movement (NREM).
Men preferred light at 6500K, while women preferred 2500K light. 6500K light resulted in faster reaction time (RT) rates for men, as indicated by cognitive performance during PVT. Light perception at 6500K in men correlated with faster reaction times and more frontal NREM slow-wave activity, or deep sleep. For men, blue light stimulus response was significantly higher than for women, indicating increased sensitivity. The results suggested that exposure to blue light before sleep may benefit attention performance and affect sleep physiology.
Inter-individual sex-related differences in light perception and vulnerability must be considered to determine the underlying causes of light effects. However, in this study, the researchers showed sex-linked distinctions that affected cognition and sleep. This provides possibilities for further research that may uncover chronic light exposure effects and mechanisms regarding light perception differences.
- S. Chellappa, et. al., Sex differences in light sensitivity impact on brightness perception, vigilant attention and sleep in humans. Scientific Reports 7, (2017). doi: 10.1038/s41598-017-13973-1.
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