Abstract:

As a dominant Zeitgeber (i.e., time giver), ambient light can affect sleep-wake behavior patterns by regulating circadian rhythm through the suprachiasmatic nuclei (SCN), which are known as the central circadian pacemaker in human beings. Artificial light at night (ALAN) has been well established to inhibit melatonin secretion, delay circadian rhythm, increase sleep disturbance, and reduce sleep quality. In contrast, field- and laboratory-based empirical studies have revealed beneficial effects of daytime light on sleep, suggesting that exposure to a high-level or larger amount of light exposure during the daytime could advance the sleep onset time, improve sleep efficiency, shorten sleep latency, decrease sleep disturbance, and improve sleep quality. However, such benefits are not always reported and the above-mentioned effects of daytime light on sleep can be governed by the parameters of daytime light (e.g. light level and spectrum), temporal factors (time of day, duration), and the light mode. The magnitude of the effects of daytime light on improving nighttime sleep can be predicted by constructing mathematical equations that couple the variables of light level, duration, and human psychological response threshold. The additional studies are required to further explore the effect of daytime light exposure on sleep.
In addition, light affects nighttime sleep through two pathways. On the one hand, light can indirectly influence the sleep-wake cycle by resetting circadian rhythms through the SCN, on the other hand, light can also directly affect sleep through the projection of melanopsin expressed by intrinsically photosensitive retinal ganglion cells (ipRGCs) to sleep- and wakefulness-related brain regions. The sleep-wake cycle of the organism is regulated by the interaction of the circadian process and sleep homeostatic process. Whereas whether daytime light exposure could affect nocturnal sleep by regulating sleep homeostatic process remains largely unknown., it’s still necessary to explore the mechanism underlying the effect of light on sleep.
Furthermore, in practical life, the de-synchronization between the natural light-dark cycle and human inner rhythms is becoming more and more pronounced, resulting in a higher risk of circadian rhythm disturbances and subsidiary health complaints such as sleep and affective disorders. Therefore, how to create a healthy lighting is still a key and attractive research topic concerned by academia and industry in the modern society where artificial light pollution is everywhere. Future research can integrate the personal physical and psychological requirements, for light and the diversity of working scenarios together to design a “human centric lighting”, which is of important economic and social value for enhancing people’s work efficiency, regulating mood, optimizing sleep, and improving well-being.

Key words: daytime light, sleep, healthy lighting, mechanism, circadian rhythm