In South Korea, approximately 5% of the population has been diagnosed with insomnia, with 17%–23% reported experiencing insomnia symptoms [1-3]. The Centers for Disease Control and Prevention (CDC) recommend that adults sleep for more than 7 hours each night to promote optimal health and well-being [4]. Conversely, sleeping less than 7 hours per night is associated with increased cardiometabolic risks such as hypertension, obesity, and diabetes mellitus, and is even associated with all-cause mortality [5-7].

Previously, it has been established that sleep duration is closely related to depression. Both an insufficient and excessive amount of sleep are known to increase the risk of depression [8]. As sleep disturbance is one of the core symptoms of major depressive disorder, sleep duration is an important factor to consider [9,10]. However, there is a lack of research on how specific sleep disturbance is reflected in different sleeping times. It is also necessary to investigate whether sleep duration is associated with depression screening tool which could help identify people with depression in advance by paying attention to sleep duration. This study purposed to find an association between sleep duration and Patient Health Questionnaire-9 (PHQ-9) which has been shown to be a valid and reliable measure for identifying depression and estimating depression severity [11].

Meanwhile, it is well known that reduced sleep duration is linked with obesity [12,13]. This is because sleep modulates neuroendocrine function and glucose metabolism. Reduced sleep duration alters metabolic and endocrine functions, leading to an increase in evening cortisol levels, decreased glucose tolerance, and decreased insulin sensitivity. Previous studies have shown that when sleep duration decreased, ghrelin, a hungerpromoting hormone, increased while leptin, a satiety-precepting hormone, decreased [14].

Moreover, previous studies have demonstrated a correlation between income level and sleep duration, indicating that lower income levels are associated with both short and long sleep duration [15-18]. Additionally, it has been found that households with lower income levels have a higher incidence of sleep duration of less than 6 hours [19]. Given that the average sleep duration in South Korea is less than 6.5 hours which is less than the worldwide average of 7.2 hours of sleep [20,21], there is a need to investigate whether this association holds true in Korea.

Despite the aforementioned importance of exploring the relationship between sleep duration and PHQ-9, body mass index (BMI), and household income, no South Korean data exists to reflect the special circumstances of the population. No large-scale studies have investigated the division of sleep duration specifically into four groups and compared the difference in PHQ-9, BMI, and income based on sleep duration. Therefore, we aimed to use a nationwide survey to investigate the difference in PHQ-9, BMI, and household income based on sleep duration. We hypothesized that PHQ-9, BMI, and household income would have differences among the groups divided by sleep duration.

This study has confirmed the association between sleep duration and PHQ-9, BMI, and household income respectively using a large sample group through the KCHS. The mean PHQ-9 score tended to increase as sleep duration decreased but the group with a sleep duration of 9 hours or more had an exceptionally higher mean PHQ-9 score than the group with a sleep duration of 7 to 9 hours. The BMI score generally decreased as the sleep duration increased. Additionally, the group with a sleep duration of 5 to 7 hours and 7 to 9 hours had higher household income than the group with a sleep duration of less than 5 hours or more than 9 hours.

This study investigated the association between sleep duration and PHQ-9 score which screens for major depressive disorder. Our study results indicated that the mean PHQ-9 score tends to increase as sleep duration decreases and the lowest mean PHQ-9 was between 7 to 9 hours. This shows that the PHQ-9 score is higher when sleep duration is either more than 9 hours or less than 7 hours, compared to the mean PHQ-9 score of all participants. Previous studies have shown that sleep disturbance is a primary symptom of major depressive disorder [9,10]. In South Korea, among 944 patients with depressive disorders from the Clinical Research Center for Depression of South Korea study, symptoms of insomnia were present in 93% of depressive patients [24]. A cut-off value of PHQ-9 has been established for depression as 10 out of a total of 27 points with reported sensitivity and specificity of 88%. PHQ-9 scores of 5, 10, 15, and 20 represent mild, moderate, moderately severe, and severe depression, respectively [11]. The Korean version of PHQ-9 has also been found to have reliability and validity in screening for depressive symptoms [25]. Our finding was consistent with previous studies that both an insufficient and excessive amount of sleep are known to increase the risk of depression [8], and this study specifically divided sleep duration of less than 7 hours into two subgroups: less than 5 hours and 5 to 7 hours.

In terms of the association between sleep duration and BMI revealed in this study, less sleep duration was associated with higher BMI. Previously, there has been an epidemiological study that revealed the association between short sleep duration and obesity [26]. Several endocrinal and metabolic pathways may explain the association between sleep duration and BMI. Short sleep duration is associated with low leptin and high ghrelin levels [14,27], which likely increase hunger and appetite and explain the increased BMI observed with short sleep duration. Additionally, short sleep duration impairs glucose metabolism and lowers insulin sensitivity [28,29]. This dysfunction also results in a behavioral pathway that makes food intake overridden without change in hormonal effect [30,31]. A previous study showed a significant U-shaped curvilinear relationship between average nightly sleep and BMI after adjustment for age and sex [27]. Therefore, it was expected that the group with a sleep duration of more than 9 hours was to have a higher mean BMI than the group with a sleep duration of 7 to 9 hours but the opposite result was observed. However, within the group sleeping less than 7 hours, it was in accordance with the previous studies [27]. Furthermore, there was no significant difference between the group of sleep duration less than 5 hours and 5 to 7 hours. This implies that sleeping less than 7 hours, rather than considering how much less, confers much more significance.

As for the results on the income level, past studies have demonstrated a correlation between income level and sleep duration, with lower income levels have been associated with both short and long sleep duration [15-18]. This study replicates previous research and found that when sleep duration is less than 5 hours or more than 9 hours, the mean value of household income was lower compared to the group with sleep duration between 5 to 9 hours.

There are several limitations that must be taken into consideration. Firstly, as it was a cross-sectional study, it does not establish a causal relationship between sleep duration and its effects on PHQ-9, BMI, and household income. Secondly, in the KCHS, sleep duration, PHQ-9, BMI, and household income were self-reported assessments. Therefore, there was a possibility of recall bias. Thirdly, the possible causes of excessive or insufficient sleep duration were not reported and considered. Possibilities of voluntary sleep restriction or sleep disorders exist [32]. Furthermore, this study may have included factors such as other psychiatric disorders, mood, medications, caffeine or alcohol consumption, and physical activity that may have confounded the results. Also, as this study sampled participants from the general population, the mean PHQ-9 score did not exceed 5 or 10, which represents mild depression to major depressive disorder. Therefore, when PHQ-9 scores are below the cut-off value, a higher PHQ-9 score cannot necessarily indicate that depression is more severe. It should be also noted that item 3 of the PHQ-9 assesses sleep disturbance, which may have influenced the overall PHQ-9 score [33].

Nonetheless, this study had strengths, such as being a community-based study with a large nationally representative sample of South Korea. Additionally, this survey was conducted as a face-to-face interview during household visits which increased the accuracy and representativeness of the results.

In conclusion, our study highlights the association between sleep duration and PHQ-9, BMI, and household income. These findings suggest that sleep duration may be correlated with an individual’s physical and mental health status. Therefore, further investigation and clinical attention is warranted.