Beyond bone health: mental and sleep health consequences of vitamin D deficiency in emerging adulthood—findings from a large-scale university study

Vitamin D deficiency (VDD) has been increasingly recognized as a potential contributor to adverse mental health outcomes, sleep disturbances, and lifestyle impairments among young populations. This study aimed to examine the associations between serum 25-hydroxyvitamin D 25(OH)D concentrations and psychological well-being, sleep quality, dietary habits, and physical activity in young adults. A total of 1,074 university students in Istanbul, Türkiye (median age: 20.0 years; IQR: 18.0-22.0) were enrolled in late winter - early spring season (February - March 2025). Participants were stratified into three groups based on their most recent serum 25(OH)D levels as reported from laboratory results within the last month: deficient (<i>n</i> = 451), insufficient (<i>n</i> = 241), and sufficient (<i>n</i> = 382). Psychosocial and behavioral outcomes were assessed using the Depression, Anxiety, and Stress Scale (DASS-21), the Bergen Insomnia Scale (BIS), the Nutrition and Exercise Behavior Scale (NEBS), and the Vitamin D Health Belief Scale (VDHBS). Multivariate analyses and odds ratios (OR) were calculated. Participants with Vitamin D deficiency (VDD) reported significantly higher scores in depression, anxiety, stress (all <i>p</i> < 0.001), insomnia (<i>p</i> < 0.001), and VDHBS (<i>p</i> = 0.01) but no significant differences in NEBS scores (<i>p</i> > 0.05). Positive correlations were found between VDD and all psychological outcomes (<i>p</i> < 0.001). Regression analysis indicated that VDD significantly increased the likelihood of insomnia (OR = 1.10), depression (OR = 1.12) and stress (OR = 1.11) (<i>p</i> < 0.05), even among individuals reporting vitamin D supplementation. These findings likely reflect recent initiation and low-dose supplementation (mean 620 ± 210 IU/day in the deficient group; 590 ± 180 IU/day in the insufficient group), insufficient to achieve physiological correction at the time of assessment. VDD is significantly associated with elevated levels of insomnia, stress, anxiety, and depressive symptoms, independent of supplementation status. These findings underscore the importance of considering serum 25(OH)D concentrations status in mental and sleep health assessments. Further longitudinal and mechanistic research is warranted to elucidate the underlying VDD with neuropsychological outcomes.