Association between depression and insomnia subtypes: a longitudinal study on the elderly in Japan.

DEPRESSION IS SUSPECTED TO BE STRONGLY ASSOCIATED WITH SLEEPING DIFFICULTIES. THE WHO STUDY ON GLOBAL BURDEN OF DISEASES PREDICTED depression would be the second greatest Burden of Disease in 2020 in developed countries.1 These countries therefore urgently need to address the issue of depression. Depression is prevalent among the aged, and it is known that its prevalence rises after the age of 50 among Japanese people.2 It is reported that insomnia can be a precursor or risk factor in depression and that depression could result in insomnia. Thus, the two diseases apparently have a bidirectional relationship.3 Furthermore, insomnia is listed as a major diagnostic feature of depression in the Diagnostic and Statistical Manual of Mental Disorders, 4th Ed (DSM-IV), attesting to the close association of the two diseases.4 This problem has attracted many researchers, and various epidemiological studies have thus far been carried out. In their pioneering longitudinal study, Ford and Kamerow5 reported that those who complained of insomnia at the baseline showed a high risk of developing major depression after one year (odds ratio [OR]: 39.8), while those relieved of insomnia by the time of a second survey showed a much lower risk (OR: 1.6). Breslau et al.6 observed young adults longitudinally for three years and found that those with a history of insomnia at the baseline had a relative risk of 4.0 of developing major depression by a second examination. Chang et al.7 conducted a long-term cohort study in which they followed college graduates up to 45 years. They reported that insomnia in young people could result in the risk of developing depression for at least 30 years. All these studies have confirmed the importance of insomnia as a risk factor of depression and the need for early detection and treatment of insomnia. Riemann and Voderholzer8 reviewed eight longitudinal studies (including the three studies mentioned above) conducted before 2000, which examined the relationship between depression and insomnia. In their review, they listed two other studies dealing with that relationship among those aged ≥ 65 years.9,10 More recently, epidemiological longitudinal studies of the association between insomnia and depression were conducted in various populations: among those ≥ 18 years old in the UK,11 those ≥ 30 years old in Norway,12 young adults in Switzerland,13 the general population in Sweden,14 and those ≥ 65 years old in South Korea.15 All of these studies, again, indicate the effect of insomnia on depression for different populations. These reports, however, did not examine the relationship of depression to the different insomnia subtypes: difficulty initiating sleep (DIS), early morning awakening (EMA), and difficulty maintaining sleep (DMS). Hartz et al.16 examined risk factors for insomnia subtypes by using a cross-sectional survey and concluded that every insomnia subtype is strongly associated with depressive symptoms. Rodin et al.17 stated EMA was more closely related to depression than three other sleep problems, including difficulty falling asleep. Although Rodin et al. collected longitudinal data to examine the relationship between sleep disturbances and depression, their analyses of the relationship were basically cross-sectional in nature. Their analyses suggested that “in the same subject, a decrease in depression is associated with a corresponding decrease in early morning awakening.” Nevertheless, two recent large-scale epidemiological studies in Japan yielded quite different results concerning the relationship between depression and subtypes of insomnia. Using the CES-D to elucidate the relationship of depression to insomnia, Kaneita et al. evaluated 24,686 people (aged ≥ 20 years) throughout Japan for depression.2 Sukegawa et al. studied 2,023 residents aged ≥ 65 years in one city using the Geriatric Depression Scale and Pittsburgh Sleep Quality Index.18 These studies, both of which used questionnaire surveys based on self-rated responses, reported that the closest connection to depression was found not with EMA or DMS, but with DIS. Being cross-sectional studies, however, they could not adequately explore the causal relationship between insomnia subtypes and the presence of depression. We therefore, employed a longitudinal survey on elderly Japanese to examine the temporal association between insomnia subtypes and the presence of depression three years later.