Comorbidity refers to simultaneous occurrence of two or more conditions that may be much more than chance association (Feinstein, 1970). Understanding specific comorbidities is crucial in many aspects. The identification of comorbidities helps to recognize the pathophysiological mechanisms of diseases, which will aid the development of new treatment strategies. The comorbidity between depression and migraine provided an insight into shared mechanisms and led to the identification of the roles of sex hormones, serotonin, and dopamine in the two disorders (Zhang et al., 2019). Recognition of comorbidities can influence diagnosis and treatment planning. Diabetic patients have altered pain sensation, making the diagnosis of coronary heart disease more difficult (Valderas et al., 2009). Coexisting cardiovascular disorders may affect the use of triptans in individuals with migraine (Buse et al., 2020). Comorbidities often contribute to the exacerbation of symptoms and increase the overall burden of a disease, thereby lowering the quality of life (QoL) (Buse et al., 2020; Lipton et al., 2000).

Depression, migraine, insomnia, and fibromyalgia (FM) have many comorbidities (Buse et al., 2020; Hertenstein et al., 2019; Kleykamp et al., 2021; Penn et al., 2019). These four disorders have a high level of mutual comorbidity. All these are prevalent among the general population and impose huge personal and social burden (Daley et al., 2009; GBD 2019 Diseases and Injuries Collaborators, 2020; Jackson et al., 2016). Additionally, they share neurochemical and neurophysiological similarities (Finan and Smith, 2013; Knijnik et al., 2016; Lan et al., 2017; Sonmez et al., 2019; Sun et al., 2021). Nevertheless, most studies have investigated the association between any two of these disorders. Only a few studies have assessed the comorbidity of three or four of these disorders (Wilson et al., 2002). Furthermore, few studies have evaluated the impact of comorbidities on symptoms. Therefore, identifying the mutual comorbidities of these four disorders would be important in reducing their burden.

The Circannual Change in Headache and Sleep (CHASE) study was an online nationwide population-based survey to assess circannual changes in headache and sleep (Kim et al., 2022b). The study included modules regarding the diagnosis of depression, migraine, insomnia, and FM, which may provide an opportunity to assess the mutual comorbidity among the four disorders. We hypothesized that depression, migraine, insomnia, and FM are comorbidities. The aim of this study was to investigate the comorbidity of these four disorders. Further, we assessed the symptom severity of these four disorders according to the number of comorbidities.