PhD Marta Benedetti on light control integrating non-visual effects

The use of daylight in buildings is important because it contributes to energy savings by reducing the use of electric lighting and space heating. It also impacts on biological functions and on well-being. Indeed, light not only allows vision, but has non-visual effects on human physiology: for instance, light is the primary time cue for the circadian entrainment of the biological rhythms to the 24-hour environmental time. This thesis is focused on two aspects: the automated control of daylighting in buildings, and the examination of its impact on non-visual functions in a semi-realistic office study.

The overall hypotheses for Marta Benedetti's thesis were that a strategy including a daylight-responsive automated control for indoor lighting combined with an advanced daylighting room design is beneficial for non-visual functions, it can maintain sound visual comfort and low electric energy demand. The main aims of this work were: 1) to set-up an automated controller for integrated daylight and electric lighting; 2) to test its effects on circadian rhythms, cognitive performance and visual comfort in a study with young participants, and 3) to evaluate its impact on energy demand. To achieve the first aim, an automated controller for venetian blinds and electric lighting was designed to provide optimal indoor lighting conditions following dynamic preset target levels of illuminance. The controller was based on real-time assessment of lighting conditions by high dynamic range vision sensors placed next to the desk at the eye level. Next, the controller was implemented in a well daylit office room (Test room) and compared with a Reference room corresponding to a standard working space without automated control. A total of 34 young participants spent five consecutive days (8h/day during normal office hours) in each of the two rooms. Lighting conditions in the room were continuously monitored, and individual light exposures were recorded by wearable devices. Several physiological outcome measures (sleep/wake times, salivary hormonal concentrations, skin temperatures) were obtained. Simultaneously, cognitive performance (on auditive computer tasks), as well as subjective comfort and well-being were repeatedly assessed. The results showed that vertical illuminance was significantly higher in the Test than in the Reference room, especially in the morning hours until the early afternoon, while visual comfort was always ensured. The higher illuminance in the first part of the day led to a faster accumulation of the daily light dose: 50% of the maximum accumulated illuminance was reached on average 50 minutes earlier in the Test than in the Reference room. Concomitantly with higher illuminance, the timing of the circadian rhythms (assessed by the onset of melatonin secretion and peripheral heat loss through the skin) was advanced by approximately 20-30 minutes in the Test room. On the other hand, participants overall performed better in the Reference room, especially in summer. The total electricity demand for space heating and lighting decreased by 9.6% in the Test room with respect to the Reference room. To summarise, these findings indicate that the lighting strategy had phase-advancing effects on circadian rhythms, which might be a useful real-setting treatment for circadian sleep-wake disorders which have become more frequent in young adults nowadays. Further work is needed to establish optimal lighting for cognitive performance across different seasons.