Sleep plays an integral role in an individual’s overall health and well being. Alvarez and Ayas have suggested that the optimal sleep duration associated with the lowest morbidity and mortality is 7-8 hours per night.¹ With the increasing demands of today’s 24-hour society, restricted or disrupted sleep (< 7 hours per 24 hours) is a widespread and serious public health problem. According to the 2009 National Sleep Foundation Survey, Americans sleep an average of 6.7 hours a night, with approximately 23% sleeping less than 7 hours per 24 hours.² In addition, the proportion of Americans sleeping less than 6 hours per 24 hours has been increasing from 12% in 1998 to 20% in 2009.² Prior research has indicated that short sleep duration is more common among individuals who are black, of low socioeconomic status, older, unmarried, who care for young children, work long hours, report alcohol use, or are overweight/obese.^3,4

Sleep durations that are longer or shorter than 7-8 hours in a 24-hour period are associated with acceleration of the aging process,⁵ increased risk of impaired glucose tolerance, diabetes,^6,7 obesity, ⁸ hypertension,⁹ injuries,¹⁰ and increased mortality.^11,12 In addition, short sleep duration adversely affects cognitive performance, learning, memory, and mental health.^13-19 Although disrupted sleep or short sleep duration associated with psychiatric disorders is usually considered as a symptom of the disease, several studies have indicated that the relationship between sleep duration and mood disorders is more complex and may be bi-directional.^20,21 Instead of being a symptom, sleep duration may be a causal factor in the development of mood disorders.^20,21

Experimental studies in rats showed that chronic short sleep duration resulted in alterations in neurotransmitter systems (e.g., serotonin and corticotropin-releasing hormone receptor systems) that have been similarly reported among individuals diagnosed with major depression.^22,23 Studies of adolescent populations have reported that teens with short sleep duration were 24% more likely to be depressed and up to three times more likely to exhibit suicidal ideation or behavior.^24,25 Similarly, studies in adult populations have found that shorter sleep duration was indicative of depressive symptoms and carried a 5-fold increased hazard of depression recurrence. ^{20, 26} Thus, while sleep disturbances are often hallmark symptoms of depressive disorders, they may also be risk factors for the development or exacerbation of depressive symptomatology.^20,26,27

Depressive disorders are associated with substantial impairment, comorbidity, poor health, and mortality.^28,29 The estimated prevalence of depressive symptoms in the US is approximately 8.7% and the lifetime prevalence of the diagnosis of depression is 15.7%.³⁰ The prevalence of current major depressive disorder is approximately 5.3%,²⁹ with rural populations having a higher prevalence than urban populations (6.1% vs. 5.2%, respectively).³¹ Rural residents are a medically vulnerable population with reduced access to healthcare services.³² They have an increased risk of health problems, such as diabetes, cardiovascular disease, obesity, and traumatic injuries.³² The greater proportion of people with poor health in rural population places them at higher risk for depression.³¹ Thus, the disabilities and impairments associated with depression may be of even greater concern in rural populations.

While a link exists between sleep duration and depression, with short sleep duration as a plausible risk factor for depressive symptoms,^12,20,22-26 this association is less established among a rural, adult population. Rural populations not only have unique demographics and lifestyles, but also unique health needs.³² Compared with urban and suburban dwellers, rural residents are more commonly employed in agriculture and small business.³³ Poverty rates are higher in rural areas compared to their metropolitan counterparts.³⁴ About one-fourth of older Americans live in rural areas and small towns, yet half of older Americans with poverty-level incomes live in such areas.³⁵ Rural residents are also more socially isolated, less likely to work in white collar occupations, live in substandard housing, have lower education levels, and less adequate access to health services.³⁴ All of the aforementioned factors could contribute to negative health outcomes, as evident in the rural health disparities in heart disease, physical inactivity, alcohol abuse and tobacco dependence, and psychiatric symptoms.^32,34,36

Recent studies have focused on the increased risk for obesity due to short sleep duration in rural populations,^37,38 yet the higher prevalence of current major depressive disorder highlights the need to examine how sleep duration may also relate to depressive symptoms in this population. ³¹ The present study was conducted to examine the relationship between sleep duration and depressive symptoms, using data from 12 rural communities in Missouri, Tennessee, and Arkansas involved in the Walk the Ozarks to Wellness Project.

The overall prevalence of elevated depressive symptoms (PHQ 2 ≥ 3) in this sample was 17.1%. The average sleep duration in this rural population is 6.9 hours (SD=1.30) and 7.4 hours (SD=1.44) for weekdays and weekends, respectively. Based on the average sleep duration for both weekdays and weekends, the proportion of study subjects who reported sleeping <7 hours per night (short sleep duration), 7-8 hours per night (optimal sleep duration), and >8 hours per night (long sleep duration) was 29.7%, 61.7%, and 8.6%, respectively. Individual with short sleep duration were more likely to be high school graduates whereas those with long sleep duration were more likely to have less than high school education, to have lower income levels, to be unemployed/retired/unable to work, and to be current smokers, compared to other participants in the study (Table 1). Study participants did not differ by race, gender, age, BMI, or histories of chronic disease and cancer by sleep duration.

Bivariate analysis indicated a U-shape association between sleep duration and elevated depressive symptoms. Specifically, compared to participants with optimal sleep duration, the crude prevalence odds of elevated depressive symptoms were 2.62 (95% CI: 1.90, 3.63) and 2.36 (95% CI: 1.43, 3.89) for short and long sleep duration, respectively. After adjusting for age, gender, race, education, employment status, income, and BMI, short sleep duration was still associated with increased odds of elevated depressive symptoms (aPOR=2.12, 95% CI: 1.49, 3.01, Table 2), compared to optimal sleep duration. Conversely, after controlling for confounders, long sleep duration was no longer associated with elevated depressive symptoms (aPOR = 1.62, 95% CI: 0.92, 2.85). Lower level of education (i.e., < high school), lower income (< $20,000, and higher BMI were also significantly associated with elevated depressive symptoms (Table 2).

Clinical and epidemiological studies have shown that sleep disturbances such as insomnia are closely linked to major depression.^42,43 Therefore, we conducted a sub-analyses on participants without insomnia symptoms for the relationship between sleep duration and depressive symptoms (n= 1,042). Table 3 indicated that after excluding individuals with insomnia symptoms, there was still a statistically significant association between short sleep duration and elevated depressive symptoms, compared to those with optimal sleep duration, after controlling for confounders (aPOR = 1.95, 95% CI: 1.28, 2.98). As in analyses including subjects with and without insomnia, Long sleep duration was not associated with elevated depressive symptoms in the adjusted model (aPOR = 1.74, 95% CI: 0.94, 3.20).

This investigation provides preliminary evidence of an association between sleep duration and elevated depressive symptoms in rural populations. After adjusting for sociodemographic factors (i.e. age, race, gender, income, education and employment status) and BMI, short sleep duration was associated with elevated depressive symptoms. Sleep disturbances such as insomnia are also risk factors and symptoms of depression.¹⁴ After excluding individuals with insomnia symptoms, we continued to observe a statistically significant association between short sleep duration and elevated depressive symptoms. Increased odds of elevated depressive symptoms were observed among those with long sleep duration compared to those with optimal sleep duration. However, this association was not statistically significant after adjusted for confounders.

In a prospective cohort study, Ganguli and associates observed that sleep complaints and short sleep duration were common among older rural residents.⁴⁴ Sleep problems among the rural residents are sometimes associated with treatable health conditions and modifiable behavioral and environmental characteristics. However, compared to their urban counterparts, individuals from rural communities have access to a smaller number and narrower range of health care providers and services and are less likely to be using these health services.⁴⁵ Hence, it is unlikely that specialized sleep medicine services are available outside major metropolitan areas to rural residents to receive the needed care in addressing their sleep complaints.

Furthermore, rural communities are exposed to circumstances, conditions, and behaviors that may put them at greater risk for depression.³⁶ These include physical inactivity, heavy alcohol consumption, fewer regular dental visits, and higher incidences of chronic illness when compared with urban residents.^31,46 Previous studies have reported that rural areas have a disproportionate share of individuals at risk of poor mental health (e.g. the elderly the chronically ill).^47,48 Furthermore, rural residents are more commonly employed in agriculture settings which are characterized by a range of physical, biological, chemical and mechanical hazards.⁴⁹ And prior research has found higher rates of depression and anxiety among farmers than the general population.^50,51 However, rural areas are generally lacking in both the level and quality of mental health services^47,48. Access to mental health services and concern for suicide stress depression and anxiety disorders were identified as major rural health concerns among state offices of rural health.⁵² Marvin and colleagues had reported that the proportion of counties that are whole-county shortage areas for mental health professionals increases from 37% among large rural counties adjacent to metropolitan areas to 76% among isolated small rural counties.⁵³ Therefore, it may be difficult for rural residents to obtain specialist care for depression. Taken together, these features of rural life and culture may have bearing on the relationship between short sleep duration and depression in rural communities different from the urban communities.

Although prior research is scant in similar populations, our findings on short sleep duration in this rural population are consistent with previous findings.^20,24,26 In a large cross-sectional study of Korean adults aged 18-64, Park and colleagues found that sleeping 5 hours or less per night was associated with mood disorders.³ Studies on pilots and officers after night duty observed that there was an increase in self-reported feelings of depressed mood, anger, frustration, tension, and anxiety during prolonged sleep deprivation.^54,55 In a small longitudinal study of patients with bipolar disorder who were free of affective disorders at baseline, Perlman and colleagues reported that shorter sleep duration predicted increased depressive symptoms over the subsequent 6 months follow-up.²⁰ Even though sleep disturbances are possibly a trait marker of depression, the medical literature is inconsistent on the directionality of this association.^56,57 Paradoxically, among individuals with mood disorders, sleep restriction can also have an antidepressant effect, ⁵⁸ underscoring the complex interaction between sleep duration and normal affect regulation.

Prior epidemiological investigations have indicated that sleep duration and mortality have a U-shaped relationship.^1,11,59,60 In a sample from the National Women’s Health Initiative, both short and long sleep duration as compared to optimal sleep duration increased the incidence of depression.⁶¹ After adjusting for covariates, our data did not support as robust a statistical association between longer sleep duration (> 8 hours per night) and elevated depressive symptoms as short sleep duration. The discrepancy in findings among studies may also be attributed to different definitions and assessment of sleep duration and depressive symptoms. Alternatively, the observed crude association between long sleep duration with elevated depressive symptoms in our study may be attributed to the socioeconomic risk factors for which we adjusted in our multivariable analysis (i.e. age, gender, race, education, employment status, income, and BMI).

Animal studies found that when subjected to chronic sleep restriction, neurotransmitter receptor systems (e.g. serotonin-1A and corticotropin-releasing hormone receptor systems²²; serotonin 5-HT dysregulation²³) and neuroendocrine stress reactivity (e.g. the HPA axis²²) were gradually altered in similar fashions to individuals diagnosed with major depression. In human samples, increased hypothalamic-pituitary-adrenocortical (HPA) activity as well as reduced growth hormone secretion have also been found in untreated depressive patients and normal individuals with altered sleep patterns.⁶² In other words, altered sleep architecture due to short sleep duration can alter chemical pathways that have been implicated in depressive patients.

Some limitations in the present study warrant mention. The direction of causality in the relationship between sleep duration and depressive symptoms cannot be established in this cross-sectional study, nor are the reasons underlying short sleep duration in our participants known. Selection bias may be present with a response rate of 65.2% in our study, with participants likely having healthier lifestyles than those not enrolled in the study. Our findings were also based on self-report data including sleep duration and depressive symptoms. Measurement error in sleep duration is possible due to lack of objective assessment of sleep duration. However, self-reported sleep duration has been found to reliably predict prospective mortality and other health outcomes.^9,11,12,63 The PHQ-2 is a valid, brief screening measure for probable clinical depression but is not a diagnostic criteria-based assessment tool for clinical depression. Lastly, our data also did not provide clinical measures of medical conditions or information on shift work, caffeine and alcohol consumption, medication usage, or prior history of depressive disorder.^26,59,62,64 Results from our study may not be generalizable to urban or suburban populations since our sample was drawn from a poor rural population.

This study suggests that short sleep duration (<7 hours per night) and depressive symptoms are common among rural populations in Missouri, Arkansas, and Tennessee. Furthermore, short sleep duration is positively associated with elevated depressive symptoms. It is estimated that the yearly economic burden of depression in the United States is approximately $83.1 billion.⁶⁵ These burdens may be of greater consequence for rural populations, as they are likely poorer, less health literate, and have inadequate access to health care, especially primary and specialty care.^64,66 Provisions could allow for improved understanding of reasons underlying short sleep duration in rural residents, as these may differ between rural and urban populations. In turn, this could lead to greater screening and preventive efforts or interventions targeted at rural residents, to avoid undue morbidity associated with depression, and interventions could focus on altering lifestyle choices to improve sleep habits and overall well-being. More research is needed to understand the risk factors associated with short sleep duration in rural communities to facilitate targeted interventions. Future prospective studies are needed to confirm our findings on the association between short sleep duration and depressive symptoms in rural populations.