## 4 Discussion
Further research is needed to identify the amount and type of light that is needed to stimulate ADL performances. Another possible explanation of the stabilisation in ADL can be the fact that the placebo light and the amber/red light provide more light in the home situation compared to baseline. The visual effects of the light can improve vision and will make it easier to perform activities. The advantage of BDL, which was a deliberate decision in this research, is that this kind of light is suitable for daily use (in the morning, afternoon, evening) and seems not to interfere with the daily activities of people with dementia living at home. The SCN is the principal circadian pacemaker in the mammalian brain, and, as such, it generates circadian rhythms in rest and activity, core body temperature, neuroendocrine function, autonomic function, memory and psychomotor performance, and a host of other behavioural and physiological processes [58]. ADL has an interactive effect on sleep with dementia [59]. Poor sleep at night can lead to further declines in activity levels or behavioural alterations during the day, potentially leading to an inability to perform basic daytime actions in some patients [23; 60]. Thereby, physical inactivity can accelerate the progression of ADL disability [22; 61], so it is important that the activity rhythm is maintained. The supplementation of light as the primary stimuli acting on the SCN may have improved its abilities to synchronise rhythms in, for example, hormones, metabolism and peripheral oscillators, which concertedly contribute to an individual's general functioning [25]. Although our study lacks evidence for a direct relation between BDL and ADL performance, it is hypothesised that the influence of light (~200 lux) could stimulate the SCN and caused the stabilisation in ADL problems found in this study. The guideline with evidence for pharmacological treatment of dementia [62] states that there are some statistically significant improvements with medication. However, drugs do not stop the progression of the disease and do have significant side effects. The advantage of the light used in this study is that there are no harmful side effects [63] when used properly and is easily implemented for a home situation. Another interesting finding is that, for example, both participants one and three rated housekeeping and finances as 'not applicable' at baseline, but later in time, they started performing these activities. Therefore, it could be that a (small) increase in ADL problems based on the visual analysis can be explained by the fact they carry out more ADL activities. When people start to perform activities that they are not familiar with, it seems likely that they need (more) support. However, the fact that they perform more ADL activities during this light intervention study is interesting to report. Some limitations have to be considered. The placebo light used in this study (~200 lux) has a higher amount of lux compared with regular daily light (baseline) the participants are exposed to. Perhaps the illumination as placebo light was too high with the result that it might have nullified positive effects of the intervention [64]. Therefore, a third phase of baseline light should have been taken into account. On the other hand, due to this study design, in some cases, a lower amount of lux instead of the suggested 1000 lux from the literature [25; 34] could also have an effect on the stabilisation of ADL problems in people with dementia. Another limitation is that it has not been objectively measured how much light (intensity and colour spectrum) the participants actually received. It was asked in the online surveys how many hours a day they actually received light from the light system (between 9 a.m. and 12 p.m. and between 7 p.m. and 11 p.m.) and how many minutes the participant spent outside.