Circadian lighting design: analysis of the properties of natural lighting in a home environment

Light is so important to human beings that its main definition from the Commission International de l’Eclairage [CIE] is linked to the human capabilities. The photometric quantities are based on human visual responses to a specific portion of the electromagnetic spectrum (380‐780nm) weighted to the visibility curve V(lambda). Furthermore, it has been scientifically established that light is useful for tuning the principal biological clock of humans to the cycles of night and day. By influencing non‐visual processes, lighting (quantity, duration and time, distribution and direction, spectra, colour temperature) has a fundamental role in physical, mental and behavioural regulation by activating the circadian system. In sync with circadian rhythm, many bodily phenomena are regulated: heart rate, body temperature, blood pressure and, consequently cognitive performances, subjective alertness, short term memory, appetite and wakefulness/sleep. The introduction of electrical lighting in the XIX century and, more recently, the explosive rise of screen technology, intending the intensive use of television, notebook, smart phones and tablets, has changed completely the relationship between human activities with lighting. Human beings are living under artificial conditions with shorten time passed in open air and poor chances to get good natural light. More than this, electrical lighting has extended the daytime activities during the nights with negative outcome of achieving the complete darkness during sleep time (spill light). These disruptions derived by modern lifestyle affect in particular the elderly population with a consequent decline of their circadian system and sleep‐related problems. Frequent nocturnal awakenings, insomnia and frequent diurnal naps are associated both with a reduction of physical health (e.g. cardiovascular problems, irregularities of the endocrine system operation, decline in immune functions) also to poor and reduced lighting reaching the retina due to the eye optical changes (e.g. cataracts). In order to understand this phenomena, the natural and artificial lighting conditions which are experienced by elderly people (in comparison with young people) in a domestic environment have been investigated through a study of a real living environment. Simulation and measurements of natural lighting conditions were performed in a case‐study apartment located in Milan through the period of one year, in different conditions of sky (clear, overcast, intermediate), in different rooms and performing different tasks. The study shows important contributions in terms of validating the importance of circadian lighting in a domestic environment which is seldom forgotten by circadian studies. The paper will describe both the methodology and the results achieved by comparing measurements and simulations. The conclusive insights would critically reflect on the lighting conditions to improve the quality of light of elderly people.