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Vitamin D deficiency is associated with physical and mental health problems, including sleep disorders. This study aimed to clarify the relationship between vitamin D deficiency and sleep and mental health problems among Japanese female university students.<\/jats:p>\n          <\/jats:sec>\n          <jats:sec>\n            <jats:title>Methods<\/jats:title>\n            <jats:p>Participants were 224 female university students. Blood levels of 25-hydroxyvitamin D [25(OH)D] were measured using liquid chromatography-tandem mass spectrometry for vitamin D assessment. Mental health was assessed using the K6. Sleep\u2013wake status as a factor related to mental health was assessed using the Athens Insomnia Scale (AIS) and Munich ChronoType Questionnaire. Loneliness was assessed using the Three-Item Loneliness Scale. Factors predicting mental health problems with a K6 score\u2009\u2265\u20095 were explored using the Mann\u2013Whitney U test, Fisher\u2019s exact probability test, and classification and regression tree (CART) analysis.<\/jats:p>\n          <\/jats:sec>\n          <jats:sec>\n            <jats:title>Results<\/jats:title>\n            <jats:p>The median (interquartile range) serum 25(OH)D concentration was 14.5 (11.8\u201318.3) ng\/mL. Of the participants, 80.8% had vitamin D deficiency (25(OH)D\u2009&lt;\u200920\u00a0ng\/mL), and 26.3% had severe vitamin D deficiency (25(OH)D\u2009&lt;\u200912\u00a0ng\/mL). In total, 41.1% had mental health problems with a K6 score of\u2009\u2265\u20095. Although there was no significant association between vitamin D deficiency and sleep\u2013wake problems, vitamin D deficiency was more prevalent among those with K6 scores\u2009\u2265\u20095 (<jats:italic>P<\/jats:italic>\u2009=\u20090.02). Compared to those with K6\u2009&lt;\u20095, those with K6\u2009\u2265\u20095 had significantly higher Loneliness and AIS scores (<jats:italic>P<\/jats:italic>\u2009&lt;\u20090.001), greater social jetlag (<jats:italic>P<\/jats:italic>\u2009=\u20090.03), shorter sleep duration on weekdays (<jats:italic>P<\/jats:italic>\u2009=\u20090.03), and lower serum 25(OH)D concentration (<jats:italic>P<\/jats:italic>\u2009=\u20090.02). In the CART analysis, the algorithm was set in the order of Loneliness score\u2009\u2265\u20096, AIS score\u2009\u2265\u20097, social jetlag\u2009\u2265\u2009150\u00a0min, and serum 25(OH)D concentration\u2009&lt;\u200914\u00a0ng\/mL, and the target accuracy (95% confidence interval: CI) was 76.5 (70.3\u201381.9)%, and sensitivity and specificity (95% CI) were 62.2 (51.4\u201372.2)% and 86.3 (79.2\u201391.6)%, respectively.<\/jats:p>\n          <\/jats:sec>\n          <jats:sec>\n            <jats:title>Conclusions<\/jats:title>\n            <jats:p>Loneliness, insomnia symptoms, social jetlag, and vitamin D deficiency were associated with mental health problems among Japanese female university students.<\/jats:p>\n          <\/jats:sec>","DOI":"10.1186\/s40101-025-00403-9","type":"journal-article","created":{"date-parts":[[2025,7,10]],"date-time":"2025-07-10T08:46:39Z","timestamp":1752137199000},"update-policy":"https:\/\/doi.org\/10.1007\/springer_crossmark_policy","source":"Crossref","is-referenced-by-count":4,"title":["Loneliness, insomnia symptoms, social jetlag, and vitamin D deficiency in relation to mental health problems in Japanese female university students: a cross-sectional study"],"prefix":"10.1186","volume":"44","author":[{"given":"Nodoka","family":"Yamashita","sequence":"first","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Shioka","family":"Ishii","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Yoriko","family":"Kotoku","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Takuya","family":"Shuo","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Hiromi","family":"Eto","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Hideaki","family":"Kondo","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"297","published-online":{"date-parts":[[2025,7,9]]},"reference":[{"issue":"4","key":"403_CR1","doi-asserted-by":"publisher","first-page":"20","DOI":"10.1038\/s12276-018-0038-9","volume":"50","author":"SM Jeon","year":"2018","unstructured":"Jeon SM, Shin EA. 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