Light behind bars: how light impacts mental health in prisons.

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This paper synthesises literature on non-visual effects of light in prison environments, drawing attention to disconnect between research evidence and current practices. This paper aims to guide prison designers and decision-makers towards lighting strategies that better support inmates' mental health and rehabilitation. A focused literature search was conducted from June to October 2024 using Google Scholar and PubMed, targeting peer-reviewed studies published between 1985 and 2024. Included studies addressed the non-visual effects of light related to circadian rhythm, mental health, artificial and natural lighting, stress, aggression and hormonal regulation. Only English-language studies providing empirical or theoretical insights relevant to confinement settings were considered. Studies focused solely on visual performance, non-human subjects or lacking health implications were excluded. Keyword combinations were refined iteratively, although some relevant interdisciplinary work may have been missed due to indexing or terminological variations. Neglecting the role of lighting in prison design contributes to poor psychological outcomes. Limited access to daylight and the overuse of artificial lighting with high blue light content disrupt circadian regulation, worsening sleep, mood and mental health. Conversely, designs that maximise daylight exposure and use adjustable artificial lighting with appropriate spectral qualities can promote emotional stability, reduce aggression and support rehabilitation. Relevant interdisciplinary studies may still have been missed due to database indexing limitations or terminological variations across fields. This paper bridges the gap between lighting design, environmental psychology and prison reform. By focusing on how light affects inmates' psychological health and rehabilitation, it offers insights into how prison design can be improved to foster well-being.