# Soriasis and Seasonality: Exploring the Genetic and Epigenetic Interactions
## 6 Seasonality of Psoriasis
They also receive more solar radiation in UV-A (315-400 nm) and visible spectrum (400-700 nm), and prolonged outdoor exposures until late evening could interfere with melatonin production. However, studies do not support the positive effect of possible higher levels of vitamin D on psoriatic patients in the summer months. [PERSON] et al. (2011) performed a meta-analysis of the influence of vitamin D supplementation on the severity of psoriasis, which could not confirm its beneficial effect. The Medical Board of the National Soriasis Foundation in the USA does not recommend oral vitamin D in psoriatic patients with normal vitamin D levels ([PERSON] et al., 1994). Considering other environmental factors, it was reported that humidity significantly influenced the epidermis structure that can contribute to seasonal deteriorations and improvements of inflammatory dermatoses such as atopic dermatitis and psoriasis ([PERSON] et al., 1994). Figure 3: Factors leading to seasonal patterns of clinical deteriorations and improvements in patients with psoriasis. Due to the changes in environmental factors, such as humidity, solar exposure, and air pollution, patients may experience a reduction or deterioration of clinical symptoms. This figure was created with BioRender.com. Another possible explanation for the phenomenon is the seasonally fluctuating expressions of several genes. [PERSON] et al. (2003) analyzed a group of patients with moderate to severe plaque psoriasis treated with etanercept or adalimumab who, after achieving a significant response to the treatment, had a temporary suspension of the treatment. The authors found that the risk of relapse and duration of the remission were related to the time of the year the treatment was stopped. The seasonality of psoriasis activity may also affect the time when the systemic treatment is started. [PERSON] et al. (2019) evaluated initiation, discontinuation, and switching biologics and systemic non-biologic drugs in 74,960 patients with psoriasis between 2016 and 2019 in the United States. The initiation of the treatment peaked in the spring months, followed by the summer, fall, and winter. Discontinuation of biologic drugs peaked in summer, and switching of biologics was highest in spring. The authors underlined that the initiation, discontinuation, and switching of the biologic treatment for psoriasis was associated with seasonality patterns, although seasonality patterns are less clear for nonbiologic systemic medications. The question remains whether the environmental factors may influence the treatment course. In a preliminary study by [PERSON] et al. (2014), sixty-two patients with moderate to severe psoriasis were evaluated at the beginning of the biological treatment, after 1, 4, and 7 months of therapy. This study involved categorizing patients into two distinct groups: those who initiated treatment during the colder months (from November to March) and those who began therapy in the warmer period (from May to September). After 1 and 4 months of treatment, it was observed that patients on IL-12/23 and IL-17 inhibitors had better improvement when the therapy was started in the summer months. Interestingly, the course of psoriasis improvement remained consistent for patients using TNF-\(\alpha\) inhibitors, irrespective of the season. The treatment outcomes after 7 months of therapy were comparable between both seasonal groups and across various types of biologics used. The research papers related to the seasonality of psoriasis are summarized in Table 3.