Background: The impact of light exposure on mental health is increasingly recognized. Modifying inpatient evening light exposure may be a low-intensity intervention for mental disorders, but few randomized controlled trials (RCTs) exist. We report a large-scale pragmatic effectiveness RCT exploring whether individuals with acute psychiatric illnesses experience additional benefits from admission to an inpatient ward where changes in the evening light exposure are integrated into the therapeutic environment. Methods and findings: All adults admitted for acute inpatient psychiatric care over eight months were randomly allocated to a ward with a blue-depleted evening light environment or a ward with standard light environment. Baseline and outcome data from individuals who provided deferred informed consent were used to analyze the primary outcome measure (differences in duration of hospitalization) and secondary measures (differences in key clinical outcomes). The Intent to Treat sample comprised 476 individuals (mean age 37; 41% were male). There were no differences in the mean duration of hospitalization (6.7 vs. 7.1 days). Inpatients exposed to the blue-depleted evening light showed higher improvement during admission (Clinical Global Impressions scale-Improvement: 0.28, 95% CI: 0.02 to 0.54; p=0.035, Number Needed to Treat for clinically meaningful improvement (NNT): 12); lower illness severity at discharge (Clinical Global Impressions Scale-Severity: -0.18, 95% CI: -0.34 to -0.02; p=0.029, NNT for mild severity at discharge: 7); and lower levels of aggressive behaviour (Broset Violence Checklist difference in predicted serious events per 100 days: -2.98; 95% CI: -4.98 to -0.99; p=0.003, NNT: 9). Incidents of harm to self or others, side effects, and patient satisfaction did not differ between the lighting conditions. Conclusions: Modifying the evening light environment in acute psychiatric hospitals according to chronobiological principles does not change duration of hospitalizations, but can have clinically significant benefits without increasing side effects, reducing patient satisfaction or requiring additional clinical staff.

The authors have declared no competing interest.

Clinicaltrials.gov NCT03788993. The trial was prospectively registered to the Regional Committee for Medical and Health Research Ethics in Central Norway, May 8th 2018 (REK 2018/946). The trial was approved on June 6th 2018. Upon approval by the Committee, the trial registration and study protocol were made publicly available in a searchable database. The RCT was listed on the Current Research Information System in Norway July 19th, 2018 (CRISTIN ID 602154). An updated protocol was submitted to the Committee on Oct 17th 2018 and was later registered on clinicaltrials.org and published. Due to logistical issues in the study group, and the need to start inclusion based on seasonal variations in daylight, the registration in clinicaltrials.org was delayed and retrospectively registered on clinicaltrials.org on Dec 28th, 2018.

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The trial was registered to the Regional Committee for Medical and Health Research Ethics in Central Norway, May 8th 2018 (REK 2018/946). The trial was approved on June 6th 2018 and all participants gave a written informed consent.

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