Restorative effects of daylight in indoor environments - A systematic literature review

Although daylight, an integral part of nature, has consistently been shown to be appreciated by and beneficial for humans, little is known about its restorative effects. This paper addresses this knowledge gap by examining current evidence for the restorative effects of daylight in indoor environments and identifying characteristics of daylight and factors affecting daylighting that are instrumental for these effects. Following PRISMA guidelines, a systematic literature search was performed on the effects of factors affecting daylighting indoors on cognitive, affective, physiological, and clinical restoration outcomes. Empirical studies with interventions on real or simulated (i.e., simulated environments or simulated daylight openings) daylight conditions where the view is omitted or controlled across conditions were eligible for our analysis. In total, 33 articles and 67 unique relationships between factors affecting daylighting and restorative outcome variables were included in our analysis. No meta-analysis could be performed, due to the diversity of manipulations and outcome measures employed, and the frequent incomplete reporting of effect sizes. Our descriptive results showed that 49% of the examined relationships reported null effects, 6% reported mixed or other (neither positive nor negative) effects, 42% reported positive restorative effects for more or more natural daylight presence in the room, and only 3% reported negative effects, suggesting a promising level of evidence supporting the restorative effects of daylight. However, not all factors affecting daylighting were found to be in agreement or equally effective in inducing restorative effects. The highest agreement was found for the effect of the orientation of the opening, with 67% of relationships tested for this factor reporting a restorative effect in spaces receiving direct sunlight compared to those with no or less sunlight. Direct sunlight presence in the room is thus the most promising daylight component inducing restorative effects, provided that there is no visual discomfort. However, the sample also has notable limitations, such as the prevalence of observational studies and scarcity of controlled experiments, as well as the high prevalence of null results. Nevertheless, the outcomes of this systematic literature review do suggest that further research on the restorative effects of daylight is promising and underline notable research gaps and future research directions in this domain.