Sleeping is crucial for health improvement, and sleep disorders can also affect physical and mental health in addition to affecting quality of life. Insomnia is a common problem in adults, and reports of its prevalence vary depending on how it is defined. Previous research revealed that the prevalence rate of insomnia in adults is 10% to 30% in Western countries.^1-3) In Japan, a study on insomnia involving 3,030 subjects over the age of 20 from the general population revealed a prevalence rate of 21.4%.⁴⁾ Cho et al.⁵⁾ conducted a telephone survey with 5,000 study subjects between 20 and 69 years, wherein insomnia was present in 22.8% (male 20.2% and female 22.8%) of the subjects.

The known risk factors of insomnia include female sex, old age, depression, snoring, low level of physical activity, presence of underlying disease, low education level and economic status, and stress. Another study affirmed that 40% of insomnia patients suffer from comorbid psychiatric disorders.⁶⁾ Depressed patients can often have accompanying insomnia, and insomnia alone increases the risk of developing depression.⁶⁾ In addition, the presence of physical illness can increase the risk of insomnia, with insomnia potentially having a secondary effect on the body as well. Of great consequence, insomnia is also related to the risk of cardiovascular disease,^7-9) with reports that decreased sleep time is linked to an increased risk of heart disease and diabetes.^10,11)

Although various studies have pointed out factors related to sleep disorders, studies investigating patients who visit primary care facilities and analyzing the relationship between insomnia and spouse-related factors are lacking. This study was conducted to evaluate the sleep quality of patients visiting primary care facilities and to identify the prevalence of sleep disorders by using the Pittsburgh Sleep Quality Index (PSQI),¹²⁾ as well as to assess related factors such as gender, age, and history of depression. Furthermore, the study sought to investigate the relevance of the spouse's sleep disorder or depression, as this could affect the sleep environment.

The purpose of this study was to investigate the factors related to the incidence of poor sleep quality in patients who visited primary care facilities. Depression was shown to be a risk factor for poor sleep quality, and non-risky drinkers were shown to have significantly lower frequencies of poor sleep quality than non-drinkers and risky drinkers. The merit of this study lies in its systematic approach to evaluating the quality of sleep in patients visiting primary care facilities, primarily through the use of survey tools. Furthermore, it is a study that once again emphasizes the greatly increased risk of depression in patients complaining of poor sleep quality. Although this is not a large-scale study, it makes an important contribution to existing knowledge in that it investigated the relationship between sleep quality and various patient factors, including lifestyle habits such as smoking, drinking, and exercising, as well as socioeconomic factors.

As seen in the results of the present study, depression remains the main risk factor for developing a sleep disorder.¹⁶⁾ Various studies have reported that insomnia is linked to depression, anxiety disorders, and substance abuse, and that insomnia is also a suicide predictor.^17-20) Along these lines, other studies state that one-third of patients who have both insomnia and mood disorders suffer from sleep disorders prior to developing mood disorders.^20,21) Potentially forming a cyclical connection, insomnia can be a symptom of depression but may also lead to depression.^17-20) Therefore, it should be recognized that patients complaining of insomnia and sleep disorders are at high risk for depression, and that appropriate evaluation and treatment for depression is necessary.

The study also showed a reduced risk of poor sleep quality in non-risky drinkers. This finding is in contrast with those of previous studies, most which indicate that alcohol consumption affects sleep by decreasing the rapid eye movement (REM) sleep phase as well as the total sleep time.²²⁾ However, according to a study conducted with Japanese subjects, 48.3% of men and 18.3% of women used alcohol as a sleep inducer,²³⁾ so an appropriate amount of alcohol might help to improve the quality of sleep.

In this study, increasing age did not lead to decreased quality of sleep. By contrast, it is generally reported that increasing age is associated with increases in sleep disorders,²⁴⁾ due to changes in sleep factors such as decreased amount of total nocturnal sleep time, increased sleep latency, decreased sleep efficiency, and decreased REM sleep.²⁴⁾ The major reason such age-based differences were not observed in the present study is likely due to the average age of the subjects being 57.3 years, with the majority being middle-aged or older. Another possible reason for this finding is that subjects were not selected from the general population, but instead from among patients who visited the department of family medicine in university hospitals for the treatment of chronic diseases such as diabetes, hypertension, cardiovascular disease, and depression.

Women are known to have a higher risk of developing sleep disorders during their menstrual period as well as during menopause.²⁵⁾ They also have a higher incidence of sleep disorders than men because women are more sensitive to stress and the influence of their external environment. In the present study, women had more frequent complaints of poor sleep quality, but this was not shown to be an independent factor in the multivariate analysis. However, various studies have reported that the prevalence of sleep disorders is 1.3 to 1.8 times higher in females than in males; similarly, the present study also showed that poor sleep quality was approximately 1.5 times more frequent in females (30.2% male and 45.8% female).

Despite reports of a connection between low socioeconomic status and sleep disorders,^5,26) the present study found no statistically significant relationship between monthly income and sleep disorders. A possible reason is that when subjects' monthly income was categorized into various levels, they differed from the monthly income characteristics of the general population. Of note, the annual average monthly income per household was 3,440,000 KRW in 2009.²⁷⁾

Among the subjects whose spouses had a sleep disorder, the prevalence of poor sleep quality was higher according to the chi-square test; however, this was not significant in the multivariate logistic regression analysis. A US study previously reported that the presence of sleep disorders in 29 couples who sleep together was affected by the relationship of the couple during the daytime.²⁸⁾ Thus, the results of the present study likely did not show a significant statistical difference because the factors that could affect the couple's relationship, such as the presence of depression in either of the spouses, were corrected for in the multivariate logistic regression analysis. Ultimately, a spouse's sleep disorder is not an independent factor for poor sleep quality; rather, it appears to affect the incidence of sleep disorders in conjunction with other factors that influence the relationship of the couple, such as depression.

Interestingly, some study studies have found that insomnia patients suffer from various concomitant physical illnesses, and that the incidence of insomnia increases with the number of physical illnesses.^20,21) However, the prevalence of poor sleep quality did not significantly differ according to concomitant disease states or medication use for diseases such as diabetes, hypertension, and hyperlipidemia. Several possible reasons exist for such a finding. First, the subjects of the studies were patients visiting primary care facilities. Second, since the number of concomitant diseases, disease severity, medication-use according to disease state, and other treatment methods varied for each study subject, the effects on sleep disorder were also different. Hence, no statistically significant relationship was found between diseases present and sleep disorders.

The present study has the following limitations. First, a subjective survey method was used instead of an objective testing method such as polysomnography. However, since the PSQI questionnaire used in the study is a useful tool that shows high concordance with polysomnography results, the results of the present study are also thought to be reliable. Second, the study used a cross-sectional design, making it challenging to prove a causal relationship between poor sleep quality and the related factors. Finally, generalization of the present results to the population at large may be limited since the subjects were mostly patients who had visited the department of family medicine in university hospitals. Nonetheless, the present findings could be helpful in identifying factors related to sleep disorders in patients visiting primary care facilities for chronic diseases such as diabetes, high-blood pressure, and hyperlipidemia.

Evaluation of patients' sleep disorders is paramount because sleep disorders influence physical and mental diseases as well as social functioning. In particular, it should be recognized that the risk of depression is higher in patients complaining of insomnia, and thus patients complaining of sleep disorders must also be assessed for depression.