Afternoon Spectral Reddening of Daylight as a Candidate Physical Cue for Sundowning Syndrome: A Computational Hypothesis

Background/Objectives: Sundowning syndrome—recurrent late-afternoon agitation, disorientation and anxiety in people with dementia—is widely attributed to disturbed light exposure. Existing accounts remain qualitative, specifying neither which property of the ambient light field changes nor when, and so yield no prediction varying with latitude, season or sky conditions. This article is a computational hypothesis: it contains no patient data and is intended to make a light-based account specific enough to be tested. Methods: We combine standard solar geometry, an analytic clear-sky radiance model and the CIE photometric and CIE S 026 melanopic weighting functions to compute the spectral composition of the light reaching an observer as a continuous function of time, for single sky directions, for vertical (corneal) planes integrated over the whole sky, for turbidities 2–6, and for an observer indoors near a window, at three European latitudes over a full year. Results: The melanopic daylight efficacy ratio at the eye falls from about 1.33 at solar noon to about 1.02 at a solar altitude of 10° and about 0.89 at 2° (turbidity 2), while modelled total illuminance changes much less (by 3–10%). The decline is a function of solar altitude alone to within 0.005 across sites and dates, is flattened but not removed by aerosol loading and, in a modelled side-lit room with interreflections, by electric light. The Sun passes 10° altitude 51–107 min before sunset, at clock times that vary by 3–6 h over the year. These are model estimates of spectral composition, least certain close to the horizon, and not measures of physiological melanopic stimulation. Conclusions: We propose that afternoon spectral reddening—rather than darkness itself—is a candidate physical cue for sundowning, possibly mediated by the diminished short-wavelength drive to melanopsin-expressing retinal ganglion cells. The proposal yields testable predictions, including an experimental design that separates spectral composition from light level, and implies that lighting interventions should be timed to local solar conditions rather than the clock.