Terrestrial illumination undergoes continuous spectral, angular, spatial, and temporal changes throughout the day, influenced by diurnal cycles and atmospheric conditions such as haze. These variations in light exposure impact human physiology and behavior, particularly in providing "zeitgebers" (time givers) for biological rhythms. We analyzed spectral light-field data collected outdoors from dawn to dusk on four days: two in Delft (sunny and cloudy) and two overcast (uniformly cloud covered) days in Newcastle. By decomposing the light field into diffuse and directional components, we identified differences in spectral composition between these for all conditions, with overcast days showing reduced variability due to increased light scattering. Our study also explored the physiological implications for circadian regulation via melanopsin and other photoreceptors. We found that α-opic illumination vectors varied with weather, their order aligning with the sequence of photoreceptor spectral peak sensitivities—from S cones, to ipRGCs, rods, M cones, and L cones—especially under sunny and cloudy skies. Analysis of hazy versus clear images revealed that haze shifts chromaticity towards blue, potentially enhancing melanopic efficiency. Although these fluctuations are large, it is plausible that they do not impact biological rhythms to the same extent as illumination variations at dawn and dusk, ensuring that the latter remain the primary drivers of circadian rhythm regulation.

The spectral shape, irradiance, direction, and diffuseness of daylight vary regularly throughout the day. The variations in illumination and their effect on the light reflected from objects may in turn provide visual information as to the time of day. We suggest that artists' color choices for paintings of outdoor scenes might convey this information and that therefore the time of day might be decoded from the colors of paintings. Here we investigate whether human viewers' estimates of the depicted time of day in paintings correlate with their image statistics, specifically chromaticity and luminance variations. We tested time-of-day perception in 17th- to 20th-century Western European paintings via two online rating experiments. In Experiment 1, viewers' ratings from seven time choices varied significantly and largely consistently across paintings but with some ambiguity between morning and evening depictions. Analysis of the relationship between image statistics and ratings revealed correlations with the perceived time of day: higher "morningness" ratings associated with higher brightness, contrast, and saturation and darker yellow/brighter blue hues; "eveningness" with lower brightness, contrast, and saturation and darker blue/brighter yellow hues. Multiple linear regressions of extracted principal components yielded a predictive model that explained 76% of the variance in time-of-day perception. In Experiment 2, viewers rated paintings as morning or evening only; rating distributions differed significantly across paintings, and image statistics predicted people's perceptions. These results suggest that artists used different color palettes and patterns to depict different times of day, and the human visual system holds consistent assumptions about the variation of natural light depicted in paintings.