Behavioral and psychological symptoms of dementia (BPSD) such as anxiety, depression, psychosis and agitation, are prevalent, often treatment resistant, resource demanding and significantly deteriorates cognition, independency, quality of life and mortality in people with dementia. The DARK.DEM trial aims at developing new diagnostics and treatment for BPSD in both specialized and municipal dementia care. The investigators will develop digital phenotyping by determining the convergent validity of data from a smartwatch against established psychometric scales for BPSD for patients admitted to NKS Olaviken gerontopsychiatric hospital. The investigators will conduct an open label single blinded randomized controlled trial to determine the effectiveness, feasibility and safety of virtual darkness as adjunctive treatment of agitation in patients with dementia admitted to the hospital. The investigators will randomize minimum 72 patients to treatment as usual (psychotropic drugs, psychological and environmental interventions) or 14 days of virtual darkness therapy, that is, exposure to light deprived of blue wavelengths from 19.00-08.00, provided in a secluded patient unit with circadian lightening. Primary outcome is 14 days change in agitation assessed with Cohen-Mansfield Agitation Inventory. Secondary outcomes are change in diurnal variation of motor activity assessed with a smartwatch and sleep monitor, other BPSD, activities of daily living, quality of life, use of psychotropic drugs, use of restraints and coercion, length of hospital stay and resource utilization. The investigators will conduct focus group interviews with managers and staff in nursing homes to explore barriers, enablers and adaptions to support implementation of the new methods in municipal dementia care