This study formed part of a large population telephone-interview study of the epidemiology of restless legs syndrome (RLS) in Korean adults.10 Insomnia was defined as either any difficulty getting to sleep or getting back to sleep after waking in the night. The frequency of insomnia was categorized as every night, most nights (5 or 6 nights) a week, several nights (2 to 4 nights) a week, once a week, less than once a week, and rarely. We excluded subjects in the last group from the analysis since they were not clinically meaningful.

A representative sample comprising 5,000 subjects aged 20 to 69 years was constituted from 42,641 telephone numbers according to a stratified, multistage random sampling method based on sex, age, economic status, and place of residence, and comprehensively interviewed using a computer-aided telephone-interview method. Samples were established based on geographical region (five Korean provinces), the population in the place of residence, sex, age (five strata: 20 to 29, 30 to 39, 40 to 49, 50 to 59, and 60 to 69 years), occupation, and income based on the 2000 Korean Census.11

Two questions on insomnia symptoms were asked: 1) Do you have any difficulty getting to sleep? and 2) If you wake up at night, do you have difficulty getting back to sleep? If the subject responded yes to either question, they were then asked to indicate the frequency within the five above-described categories.

This survey was conducted by trained interviewers from Taylor Nelson Sofres. Data were collected using a software package designed specially for this type of computer-assisted telephone survey.

All of the analyses were performed using SPSS 13.0 for Windows (SPSS, Chicago, III). The sampling error was ±1.39 points for a 95% confidence interval. We analyzed the data using the chi-square test for categorical data. Logistic regression analysis was used to compute the risk factors associated with insomnia symptoms.

The 5,000 subjects comprised 2,470 (49.4%) men and 2,530 (50.6%) women, with 1,075, 1,235, 1,155, 784, and 751 of the subjects aged 20-29, 30-39, 40-49, 50-59, and 60-69 years, respectively. The percentages of the subjects from the Seoul/Gyeonggi/Gangwon, Busan/Gyeongnam, Daegu/Gyeongbuk, Gwangju/Jeonla/Jeju, and Daejeon/Chungcheong provinces were 51.0%, 16.4%, 10.8%, 11.8%, and 10.0%, respectively. The percentages of the subjects in the occupation categories of agriculture/fishery/forestry, self-employed, blue-collar, white-collar, housewife, student, and unemployed/other were 5.4%, 12.1%, 10.1%, 19.5%, 32.3%, 10.3%, and 10.3%, respectively. The percentages of the subjects with monthly incomes of <1.5, 1.5-3.0, 3.0-4.5, and >4.5 million Korean won were 25.7%, 30.2%, 10.5%, and 9.3%, respectively; 24.3% of them refused to state their income (Table 1). The sex, age, economic-status, and place-of-residence distributions of the study subjects were representative of the general Korean adult population.

More than one fifth (n=1,141, 22.8%) of the 5,000 subjects complained of insomnia, with the prevalence being significantly higher in women (25.3%) than in men (20.2%, p<0.001). This gender difference was similar for younger and older groups (Fig. 1). Difficulty maintaining sleep was reported by 18.3% of the subjects (19.8% of the women and 16.7% of the men) and difficulty initiating sleep was reported by 11.4% of the subjects (14.0% of the women and 8.8% of the men). Chi-square tests indicated significant differences in the initiation and maintenance of sleep. The prevalence of insomnia increased significantly with age (p<0.001), peaking at 34.6% in those aged 60-69 years (Fig. 1), and was higher in those working in agriculture/fishery/forestry, housewives, and those on a lower monthly income (Table 1), but it did not differ with the place of residence. Moreover, logistic regression revealed that the prevalence of insomnia relative to being aged 20-29 years was significantly higher in those aged 40-49 years (OR=1.590, 95% CI=1.224-2.066, p<0.01), 50-59 years (OR=2.176, 95% CI=1.647-2.875, p<0.001), and 60-69 years (OR=2.368, 95% CI=1.762-3.182, p<0.001). The prevalence of insomnia relative to having a monthly income of <1.5 million Korean won was lower in those with monthly incomes of 1.5-3.0 million Korean won (OR=0.663, 95% CI=0.544-0.807, p<0.001) and >4.5 million Korean won (OR=0.689, 95% CI=0.523-0.906, p<0.01)(Table 2).

Difficulty initiating sleep occurred every night, on most nights (5 or 6 nights) a week, on several nights a week, once a week, and less than once a week in 2.6%, 1.9%, 4.0%, 2.0%, and 0.9% of the subjects, respectively; the corresponding prevalence rates for difficulty maintaining sleep were 2.5%, 2.2%, 6.5%, 4.0%, and 3.1%.

The prevalence of severe insomnia (occurring on least two nights per week) was 14.9%, with 8.6% reporting difficulty initiating sleep and 11.2% reporting difficulty maintaining sleep. The prevalence of severe insomnia was also significantly higher in women (17.0%) than in men (12.8%, p<0.001), and increased significantly with age (p<0.001)(Fig. 2).