# Soriasis and Seasonality: Exploring the Genetic and Epigenetic Interactions
## 6 Seasonality of Psoriasis  
Clinically observed seasonality of medical conditions is a well-known phenomenon [183]. The relationship between environmental factors presenting seasonal patterns, genetic back ground, epigenetic modifications, and inflammatory reactions may influence the activity of psoriasis symptoms (Figure 2). The differences in the activity scores of the diseases seen among different countries during the year may not solely be influenced by environmental factors but also the cultural activities and festivities. For example, intermittent circadian fasting (Ramadal), a common Muslim practice, seemed beneficial in patients with psoriasis and psoriatic arthritis [184; 185]. There is a common belief that in most patients with psoriasis, skin lesions improve in warmer months of the year [183; 186; 187; 188; 189; 190; 191; 192] (Figure 3). However, not all literature data support this statement (see Table 3 for more details) [193; 194; 195; 196]. [PERSON] et al. [187] examined the seasonality of the hospitalizations at a dermatological ward, with no differences observed regarding patients with psoriasis. A study by [PERSON] et al. [188] revealed that 77% of patients with psoriasis reported seasonal changes in the disease activity with exacerbations in winter (67%) and summer months (24%). [PERSON] et al. [197] conducted a questionnaire study in northern Germany. Five hundred thirty-six patients were self-classifying into four types of psoriasis intensity during a year: type 1--stable disease course with no variation depending on the season (40.86%); type 2--unstable disease course with no variation depending on the season (22.57%); type 3--increased flare incident rate in winter (30.6%); and type 4--increased flare incident rate in summer (5.97%). In a retrospective study of 20,270 Chinese patients with psoriasis, the disease duration, hyperlipidemia, and smoking were associated with severe psoriasis in autumn and winter [198]. In this study, age and occupations with extensive sunlight  
Figure 2: Relation between environmental factors, genetic background, epigenetic modifications, and inflammatory reactions leading to deterioration of psoriatic skin symptoms. Environmental factors presenting seasonal patterns, such as solar radiation, humidity, air pollution, and circadian rhythm disturbances, influence genetic background and inflammatory drive within the skin [133; 139; 157; 160; 167; 77; 128]. Genes associated with psoriasis may be altered by environmental factors through epigenetic modifications [54; 56; 58; 162; 196]. Inflammation also causes a shift in gene expression, which leads to a positive feedback loop [66]. Inflammatory reactions cause hyperkeratinization and skin symptoms [66]. This figure was created with BioRender.com. exposure were negatively associated with seasonal psoriasis exacerbation. Considering the findings of [PERSON] et al. (199), only 30% of psoriatic patients in northern and central Europe reported improvement in their disease activity in the summer months; however, other factors could be responsible for the results obtained in this study. A survey study conducted on 1080 Polish patients with PsV revealed that seasonal changes were reported to have a considerable impact on the psoriasis disease course in 45.09% of patients ([PERSON] et al., 1994). Internet searches connected with psoriasis and its treatment also showed statistically significant seasonality peak in late winter ([PERSON] et al., 1994). On the other hand, [PERSON] et al. (1994) did not observe any difference in seasonal frequency in the number of patients with psoriasis using health services in Japan. Outdoor conditions for the mid-autumn/winter and mid-spring/summer seasons are very different in Europe, with higher temperatures and stronger solar radiation in the latter period ([PERSON] et al., 1994). In mid-spring/summer, it is evident that the patients have a chance to synthesize large amounts of vitamin D during outdoor activities due to UV-B exposure.