Abstract: Light is, clearly, a key to life on Earth and light, equally clearly, determines biological rhythmicity in organisms. Light does the latter by setting internal or endogenous clocks which allow a multitude of species, including man, to adjust their lives to changing external or environmental conditions. Critical changes over time occur from day to night and throughout the year. In this paper, we sum up how visible light provides electromagnetic information about environmental “time” via the ocular interface of newly discovered photoreceptive cells to a master clock in our brain, viz the suprachiasmatic nuclei [SCN], and how the SCN translate this input, with melatonin as a key biologic intermediary, into endogenous or biological time. We summarize experimental and epidemiological evidence suggesting how chronodisruption, a relevant disturbance of the temporal organization or order of physiology, endocrinology, metabolism and behaviour, is probably detrimental for human beings. On the basis of our synthesis, and in line with suggestions by other researchers voiced decades ago, light must, functionally, be considered as a drug equivalent. In this vein, the very timing, quality (wavelength), quantity (dose) and side effects, including chronodisruption, of light exposures can be critically important for health and disease in man. As a promising means to foster public health, we advocate an appropriate balance of exposures to the key Zeitgeber light in terms of “light hygiene”, implying strong and appropriate rather than weak and confusing temporal information. This focus on “light hygiene”, and thus on the key Zeitgeber light, does not mean to ignore that there are multiple entrainment pathways for our circadian clocks. Indeed, when dealing with light, chronodisruption and a multitude of adverse health effects, we ultimately need to consider Zeitgeber cues, and their possible interplay, beyond light alone. Confusions of the temporal programmes in humans can also stem from physical and social activities, stress and facets of food intake. And yet, since light possesses a rather unique and exclusive Zeitgeber role and in view of its ubiquitous nature, a specific, preventative focus on “light hygiene”, as a contribution to a general “Zeitgeber hygiene”, is warranted.

Abstract: BACKGROUND: Previous studies have shown that the human circadian system is maximally sensitive to short-wavelength (blue) light. Whether this sensitivity can be utilized to increase the size of phase shifts using light boxes and protocols designed for practical settings is not known. We assessed whether bright polychromatic lamps enriched in the short-wavelength portion of the visible light spectrum could produce larger phase advances than standard bright white lamps. METHODS: Twenty-two healthy young adults received either a bright white or bright blue-enriched 2-h phase advancing light pulse upon awakening on each of four treatment days. On the first treatment day the light pulse began 8h after the dim light melatonin onset (DLMO), on average about 2h before baseline wake time. On each subsequent day, light treatment began 1h earlier than the previous day, and the sleep schedule was also advanced. RESULTS: Phase advances of the DLMO for the blue-enriched (92+/-78 min, n=12) and white groups (76+/-45 min, n=10) were not significantly different. CONCLUSION: Bright blue-enriched polychromatic light is no more effective than standard bright light therapy for phase advancing circadian rhythms at commonly used therapeutic light levels.