Schizophrenia and other psychotic disorders are lifelong neuropsychiatric conditions that affect 2–3% of the population (1, 2). The main characteristics of psychosis are hallucinations (false or distorted sensory experience that appear to be real), delusions (false beliefs), and changes in the form and content of thoughts and speech. Symptoms typically begin in late adolescence or early adulthood. Schizophrenia has frequently been associated with abnormalities in the dopaminergic system (3).

Vitamin D is an essential nutrient that plays an important role in many biochemical functions that contribute to aging, including bone renewal, cell proliferation, hormone balance, and cardiovascular and glucose metabolism (4). Of particular interest is accumulating evidence for the role of vitamin D in the brain (5, 6). Animal studies have shown that deficiency of prenatal vitamin D is associated with persistent changes in adult brain structure, neurochemistry, and behavior (5, 7). Evidence now links low vitamin D levels with reduced cognitive performance (8), increased risk for cancer, diabetes, cardiovascular disease, and premature death (9, 10).

The main source of vitamin D for humans is from its dermal synthesis by exposure to sunlight (11, 12), which provides up to 90% in the form of vitamin D3 (cholecalciferol) with the remainder coming from food in the form of vitamin D3 or vitamin D2 (ergocalciferol) (6, 9, 13, 14).

In order to convert 7-dehydrocholesterol to pre-vitamin D3, the skin is penetrated by solar ultraviolet B radiation (wavelength between 290 and 315 nm), which then is isomerized by heat to vitamin D3 before being carried, bound to vitamin D-binding protein, to the liver where it is converted into the main metabolite 25-hydroxyvitamin D3. Intoxication by production of excess amounts of vitamin D3 is not caused by prolonged sun exposure. Too much UV radiation from the sun does not produce excess amounts of vitamin D3 (15). Moreover, it has been argued that prolonged exposure can break down vitamin D, reducing health benefits and increasing risk of skin cancer (13). Therefore, it has been suggested that moderate sun exposure is equally effective as high sun exposure for previtamin D production (1, 5).

Murri et al. reviewed seven studies on vitamin D and psychosis and concluded that individuals with psychosis showed lower vitamin D levels than healthy controls (16), and a comprehensive meta-analysis of observation studies suggested a strong relationship between vitamin D deficiency and schizophrenia (17) Vitamin D has been found to modulate dopamine neurotransmission (5), and several studies reported an association between early life vitamin D deficiency and later psychotic disorder. Vitamin D deficiency around birth has been associated with increased risk for later schizophrenia in a Danish population-based study (18). McGrath and colleagues reported that “both low and high concentrations of neonatal vitamin D were associated with increased risk of schizophrenia.” Similarly, absence of supplementation with vitamin D before the age of one was found to be associated with later schizophrenia in males (19). However, other studies have failed to find a statistically significant association (20–22). While there are many studies reporting the association between vitamin D levels in the blood samples (18, 19), there is still a lack of studies on sun exposure, the main source of vitamin D in most countries. Only one study has reported on the relationship between sunlight exposure and schizophrenia incidence, but found no association. This study was limited in power and did not measure sun exposure at the individual level but instead used average sun exposure for a larger geographic area (23). Thus, the association between vitamin D, sun exposure, and psychosis remains debated. Previous research suggests that subclinical psychotic experiences (PEs) might be on a continuum with psychotic disorders. PEs are present in 5–15% of individuals from the general population (1, 24) and share risk factors with psychotic disorders, including social adversity (e.g., low socioeconomic status (SES), urbanicity) (24) and cognitive impairment (25). Imaging studies have also revealed pathophysiological overlaps between psychotic disorders and subclinical psychosis (26–28). Moreover, reporting PEs in early life is found to be related to increased risk of later psychotic disorders. PEs have also been associated with several other mental health disorders such as, anxiety, depression, substance misuse, suicide risk, and self-harm (24, 29–32).

Because PEs are common and associated with a range of adverse health outcomes (1, 24), it is important to study them. Moreover, since individuals with PEs generally do not receive antipsychotic medications, potential confounding by medication is avoided.

Given the limited research, the aim of this study is to investigate the association between adult life sun exposure (as a proxy for vitamin D status) and later PEs. Using a large, prospective, population-based, cohort study, we tested the hypothesis that sun exposure behaviors are associated with PEs.

The study population originates from The Swedish Women’s Lifestyle and Health cohort. The cohort has been described in detail elsewhere (33). Briefly, the cohort included women aged 29–49 years residing in the Uppsala Health Care Region in Sweden during 1991 and 1992, of 96,000 randomly selected from four age groups (30–34, 35–39, 40–44, and 45–49 years) and invited to participate. A total of 49,261 women (52%) returned a completed questionnaire, which included demographic data and information on sun exposure behavior in different periods of life.

In 2002/2003, a follow-up study was initiated, and women who had responded to the 1991/1992 questionnaire were alive and living in Sweden in October 2002 were contacted. Since 1991/1992, 688 women were deceased and 491 women had emigrated. 47,859 women were invited to participate. 34,415 women (72%) returned the questionnaire, which included PEs using the Community Assessment of Psychic Experiences (CAPE) scale. The Swedish Data Inspection Board and the regional Ethical Committee approved the study (Figure 1).

The population consisted of 34,297 women who reported data on the CAPE questionnaire at follow-up stage of the study in 2003. There were 1,252 women with missing values on sun exposure and on the confounder (years in education). When filling in the follow-up questionnaire, the mean age of women was 40.4, SD = 5.7. The average number of years in education was 12.4 years SD = 3.0 (Table 1).

In the Nordic countries, as elsewhere, sun exposure is by far the most important source of vitamin D. In a population-based cohort of over 30,000 women, we found an association between cumulative measures of sun exposure at ages 10–39 years and positive PEs later in life. We observed a U-shaped association where women with low sun exposure as well as women with high sun exposure reported more PEs compared to women with an intermediate level of sun exposure.

The U-shaped association is consistent with previous studies on vitamin D and health outcomes (18). The non-linear association presented in this study might suggest against a possible causal link. However, the link between sun exposure and health outcomes is often reported as non-linear with a balanced exposure showing higher effects (49, 15).

An earlier study by Hedelin et al. using the same cohort of women showed that vitamin D intake from the diet was statistically significantly associated with a decreased relative risk of both medium and high levels of psychotic symptoms (50). The conflicting finding of the present study could possibly be attributed to any of the following: (1) different measures were used as proxies for vitamin D—the present study only considered the sun exposure variables (and not diet) as a main source for vitamin D levels (9, 13, 14); (2) the focus of the present study was detailed assessment of the entire distribution of the CAPE scale whereas Hedelin et al. reported results for the categorized CAPE scale (low, middle, high level). We believe that this categorization of responses could exclude important information. Nevertheless, both studies using the same cohort and the same underlying CAPE scale reported a positive association. One conclusion that can be drawn from our results is that an intermediate sun exposure could be protective for PEs. The study by Hedelin et al. showed a J-shaped association, with reduced risk for an intermediate intake of vitamin D (fish intake) (13).

Consequently, a major strength of our study is the assessment of sun exposure during different periods of life. The information on sun exposure habits was gathered at the start of the study, which minimizes the risk of reverse causality. Still, our study possibly includes non-differential (or random) measurement error since sun exposure information was collected retrospectively, which might attenuate the examined associations.

Another major strength of this study is use of an unselected, large, population-based cohort randomly drawn from the Swedish National Population Register and followed for over 10 years.

In the present study, we performed a quantile regression. This method allows for comparisons not only of mean values between two groups but also for group comparisons in the tails of a data distribution. This is often of interest for epidemiologists who may want to compare the point where the top 10% of the study participants are found rather than where the average degree of response is found. This method is important when calculating a non-linear risk factor (45).

Our study has several limitations. We had no biological data on vitamin D levels, which could have established a clear link to vitamin D, although it is well known that sun exposure is the most important determinant of vitamin D status (9, 13). Additionally, the assumption was made that sun exposure habits did not change over time and consequently, information from one assessment alone was used in the models. This is a common assumption in cohort studies and tends to underestimate risk. However, the questionnaire did take changes into consideration, as it asked about exposure in different decades.

The current study was based on multiple exposure factors (several sun exposure variables) as proxies for vitamin D level and their relationship with PEs. The estimates were adjusted for confounding effects and each exposure variable was assessed independently. Still, our results should be interpreted cautiously. Even well-designed observational studies can be influenced by residual confounding. There may also be several unknown factors that we did not take into consideration in our study, which could influence PEs and be related to sun exposure. Another limitation is that there is only a small number of published studies reporting the association between questionnaires-based sun exposure and vitamin D. For example, Glanz et al. confirmed the validity of self-reports for UV radiation and a Norwegian study by Brustad et al. found that questionnaire-based reports of sun exposure was considered a strong predictor of vitamin D (51). The same study further reported that “staying at more southern latitudes during the previous summer was also positively associated with vitamin D in the blood” (51) (sun exposure self-reports included in the present study consist of questions related to time spent on sunny holidays in Southern latitudes).

As suggested by The American Cancer Society, excessive sun exposure is considered as potential risk factor for the malignant melanoma. A previous study using the same cohort as ours linked sun exposure to malignant melanoma (43).

We did not have data on the full CAPE scale, but only the positive symptoms. Moreover, the degree of distress associated with PEs was not included in the current study, which could have allowed a more detailed analysis on items and dimensions. The CAPE scale of PEs has been reported to be correlated with general psychopathology measures, including depression. The association between the positive and the depressive dimension in CAPE, which we could not include for practical reasons, is fairly low when distress associated with positive symptoms is held constant (r = 0.25) (52). As a consequence, the dimension of positive symptoms is believed to be considered an independent dimension. Further, data on PEs were not measured at baseline therefore reverse causality could be a potential explanation of findings.

In terms of generalizability of the present study, males were not included and previous reports suggest a gender difference in the incidence of psychosis. However, in general population samples, PEs are found to be equally distributed among males and females (34, 53).

Both low and high sun exposure is associated with increased levels of positive Psychotic experiences. Previous studies have reported associations between vitamin D and increased risk for health problems, such as schizophrenia, as well as clinical symptoms in patients with mental health problems. However, there is a lack of previous studies on how sun exposure, the main source of vitamin D, influences Psychotic experiences in the general population. This study contributes to the literature by showing, in the general population of Swedish women, that sun exposure has a U-shaped association with positive Psychotic experiences.

This study was carried out in accordance with the recommendations of “Institutional/Ethics Review Boards at Uppsala University and Karolinska Institutet” with written informed consent from all subjects. All subjects gave written informed consent in accordance with the Declaration of Helsinki. The protocol was approved by the “Ethics Review Boards at Uppsala University and Karolinska Institutet.”

IP was responsible for the conception of the study, interpretation of the data, manuscript design, and statistical analysis. SS was responsible for the manuscript design, statistical analysis and data interpretation. AR was responsible for the manuscript drafting, data interpretation, and revision for important intellectual content. AD and RS were responsible for providing important intellectual content throughout the manuscript’s production and for approval of the final version. EW was responsible for the study design, manuscript drafting, and revision for important intellectual content and for approval of the final version.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.