Non-visual effects of light on human circadian physiology and neurobehavioral performance

Light is of crucial importance for human circadian rhythms. In fact, light exposure allows for resetting individual biological rhythms to the 24-h day. Besides its synchronizing effects, light also acts on different behavioural and physiological variables. The overarching aim of this thesis was to investigate the effect of different light properties, such as intensity, wavelength, duration, timing and dynamics, on neurobehavioral performance and circadian physiology, and possible inter-individual differences.
In the first part, we investigated the effect of three morning light settings (dim light, DL < 8 lux; monochromatic blue light, mBL at 100 lux; and dawn simulation light, DsL increasing from 0 to 250 lux) in 17 young participants (20-35 years old), after two nights of 6-h sleep restriction, on alertness, well-being, melatonin and cortisol profiles and cognitive performance. We found that exposure to artificial morning DsL improved subjective perception of well-being and mood, as well as cognitive performance across the day compared to DL and mBL. Only morning mBL induced a phase advance of the circadian profile of melatonin, thus impacting on the circadian system.
In the second part, we compared the effect of three light settings (dim light, DL <8 lux; polychromatic white light, WL at 250 lux; and blue- enriched polychromatic white light, BL at 250 lux) on subjective sleepiness and physiological variables during a 40-h sleep deprivation protocol. Inter-individual differences were investigated with respect to (1) age, by enrolling a cohort of 26 young (20-35 years old) and 12 older participants (55-75 years old); and (2) genetic predisposition (polymorphism in clock gene Period3), by enrolling 8 young PER34/4 and 8 young PER35/5 participants. Accordingly, the age-related effects were such that exposure to BL and WL improved subjective sleepiness in both age groups, while melatonin suppression was only detectable in the young, with a more pronounced effect under BL, and not in the older. Only the blue-enriched light modified cortisol levels, with a decrease in the young and an increase in the older. Both lights had a contrary effect depending on the age of the participant in regard to skin temperature and motor activity. With respect to the genetic predisposition, exposure to BL and WL suppressed melatonin in both groups, with a stronger effect under BL in the PER35/5. However, we showed a significant alerting response, a better well-being, and a decrease in cortisol levels only in the short allele carriers (PER34/4). In contrast, cognitive performance was decreased only in PER35/5 under WL.
In conclusion, depending on the purpose to use non-visual effects of light, either DsL or mDL can be used to improve subjective mood and cognitive performance or to shift internal rhythms, respectively. In a broader perspective, the use of moderately bright light in night work and shift work settings, where constant light levels are very common, may differ across shift workers given their age and their genetic predisposition.