United States Department of Transportation
i
Light Isn’t Just for Vision Anymore: Implications for Transportation Safety (Part II)
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2009-07-06
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[PDF-397.64 KB]
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Abstract:Humans are a diurnal species, programmed to be awake during the day and asleep at night. Therefore, it is not surprising that sleepiness plays an important role in vehicles accidents. In principle, light can be used to increase alertness at night and thereby possibly reduce sleep-related traffic accidents. We recruited 16 subjects to participate in a within-subjects experiment to investigate whether exposures to low levels of blue light, predicted to reliably stimulate the circadian system, would positively affect alertness and night-time driving performance without significantly decreasing visibility. Subjects participated in the study during the day and at night. During each session, subjects were asked to perform a driving simulator task for 3.5 hours while exposed, in a counterbalanced manner, to two levels of blue light (6.5-8.5 lux and 2.5-4.5 lux at the cornea of a 436 nm light) with a dim red light exposure (<2 lux at the cornea of a 630 nm light) in between. In addition to measuring driving performance (velocity, throttle and steering), we collected saliva samples for melatonin assay, objective alertness (EEG and ECG) and self-report sleepiness (Karolinska Sleepiness Scale). Disability glare calculations were performed. Neither the high nor the low level of blue light suppressed nocturnal melatonin production. Results did not show a significant effect of light on alpha power, beta power and heart rate. Throttle was significantly reduced after exposure to the higher blue light level, but no other effects on driving performance were observed. Subjects felt sleepier at night than during the day, but this increase in subjective sleepiness did not result in poorer driving performance. A higher threshold of the circadian system to shorter wavelengths of light was found. An important next step would be to better understand the retinal mechanisms associated with this lower-than-expected response by the circadian system to shorter-wavelength blue light. It is also recommended that a more formal assessment of the impact of disability glare on driving visibility at night from a longer wavelength blue light (e.g., 470 nm), known to positively impact the circadian system, be made.
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