The purpose of this study was to investigate the prevalence and correlation of short and long sleep duration among individuals aged from 18 to 85 years old in Guangdong province in China. To the authors’ knowledge, it is the first study using complex stratified sampling method in southern China, aiming at examining whether sleep duration was associated with sociodemographic characteristics, mental health and chronic diseases. We tested our initial hypothesis and the results supported partially for the hypothesis. People who had lower income, who were ex- or current drinkers, who were overweight, and who had depressive symptoms were at higher possibility of having short sleep duration. Furthermore, short sleepers were more likely to have anemia, hyperlipidemia and low back pain, and long sleepers had close relationship with malignant tumor.
Firstly, seeing from the side of overall length of sleep duration, the average sleep duration of this study was 6.75h, which was lower than in a similar-scale cross-sectional study in Jilin province in China (7.3h),[13] a community-based study in Beijing (7.8h)[26] and a Singapore Chinese study (7.0h).[27] Considering multiple factors, spatial regional difference, cultural characteristics, the mean age of sampling population, sampling method and inconsistency of definition of short and long sleep duration were the main reasons of the inconsistency of the results. In the study conducted in Singapore, only elderly people aged more than 60 years old were included and change in sleep was categorized as short sleep duration which was < 6h, long sleep duration which was > 8 h and stable sleep duration which was 6-8h.[27] The study[13] conducted in Jilin province in China had similar sample size and sample method comparing to this study. However, the results still showed diversity in distinct areas of the same country. Guangdong province belongs to southern coastal China, while Jilin province locates in northeast China. So Guangdong province is much warmer than Jilin province during most of the time. Diet habit, economy level, urbanization level, and rhythm of life also vary between the two provinces, which may contribute to the difference of the residents’ sleep duration.
Regarding sociodemographic characteristics, low-income earners reported more short sleep in our study. Short sleep duration had been suggested to be positively associated with low income level in western and east Asia studies.[28, 29] In China, a monthly salary below 6000CNY is at relatively low income level. Industrial workers, left-behind farmers, migrant workers, disabled, elderly and so on accounted for the vast majority of low-income earners, which are also vulnerable groups of short sleep. Most of the low-income earners are blue-collar workers, whom have more physical activities, long working time and irregular working rhythm. [12] This may explain the increasing odd ratio of short sleep duration in low-income earners. Income level and educational level are both indicators of socioeconomic position, which is closely associated with living pressure and sleep duration.[30] However, we did not find any differences in education level factor, which was consistent with the Jilin study, [13] but inconsistent with some of previous studies in western countries. [31, 32] In developed countries, people who received higher education might have more satisfied employment opportunities supporting higher life quality, whereas those who received lower education may feel stressful each day because of great stress from life and have less sleep time. However, the pattern is different in China, where people with higher education might also have stress in work and in life. As for marital status, no significant results between single, married and divorced samples were found. This suggests that married status have less effect on people’s sleep duration. The results were inconsistent with another study[33], which found single individuals were more likely to report very short sleep. It is possible that single, divorced or married people have similar amount of daily activities. Marriage is becoming decreasingly important for people of all ages.
Unhealthy lifestyle factors contribute certain possibilities to short sleep duration. In our study, associations were found between short sleep and drinking as well as smoking. The first possible reason is that for alcohol users, drinking may mediate their sleep homeostasis through adenosine and the wake-inducing cholinergic neurons in the basal forebrain.[34] For healthy non-alcoholics, although the NREM of the first part of sleep can be promoted and consolidated, the second part of the sleep can be disrupted.[34] And for alcoholics they have problems in controlling drinking, being preoccupied with alcohol, continuing to use alcohol even when it causes problems and having to drink more to get the same effect. So drinking can alter the sleep architecture, lead to insomnia and difficulty maintaining sleep. For the relationship between smoking and sleep duration, significant link between cigarette use and sleep duration has been reported in adolescence[35]. Our study illustrated a link between smoking and short sleep in adults, which added the proof of this relationship in adult age group. Inadequate sleep duration has been particularly linked with substance use, especially alcohol and cigarette. People who were overweight had high risk of short sleep. Weight gain is associated with short sleep duration, which is consistent with a cross-sectional study conducted in Taiwan of China.[36] Given that energy consumption is abnormal in people suffer from obesity, energy balance should be considered into account. Leptin and ghrelin are hormones recognized to have influence on energy balance,[37] which play roles in mediating appetite and energy balance.[37]
In the adjusted analysis, this study only found that anemia, hyperlipidemia and low back pain were associated with short sleep while malignant tumor were associated with long sleep. Chronic disease and sleep duration are bidirectionally connected with each other.[27] Short sleep can be a consequence of chronic disease, or a marker of chronic disease. In this study, a strong link between short sleep and anemia was found. Consistent with prior findings, sleep duration is associated with lower hemoglobin levels.[38] Fatigue is an important factor mediating the link between short sleep and anemia. Studies have noted that patients with anemia may have fatigue,[39, 40] and that fatigue could lead to physical activities decreased directly and affect the circadian rhythms and sleep duration.[41] Consideration of anemia has been recommended as part of the clinical evaluation of patients with insomnia. As for hyperlipidemia, long sleep duration has been reported to be associated with higher risk of hyperlipidemia,[42] while some studies observed positive association between short sleep duration and high TG or TC levels[43]. In our study, short sleeping habits may predispose individuals to hyperlipidemia risk. Other metabolic diseases like hypertension and diabetes mellitus have been mentioned to be associated with aberrant sleep duration in previous epidemiologic studies.[4, 44] However, after we adjusted confounders, hypertension and diabetes mellitus have no significant associations with aberrant sleep duration. Individuals with low back pain were at high risk of having short sleep. Insomnia was commonly reported by patients with low back pain.[45] Since pain experience is closely associated with fatigue, we speculate that back pain and fatigue experienced by patients were strong factors for predicting short sleep.
Malignant tumor had positive relationship with long sleep, which was also mentioned in the study conducted in the UK and USA.[46] Malignant tumor is a disease of high complexity. The relationship between long sleep and malignant tumor incidence may vary depending on the course and types of the disease.[46] Interestingly, in terms of cancer subtypes, long sleep duration was found to be associated with increased risk for colorectal cancer and decreased risk for hormone related cancer.[47] Previous results were partly controversial with our results. Up to date, the mechanism which can explain the effect of long sleep duration on risk of malignant tumor is still obscure. Most of the studies found short sleepers had higher risk of having malignant tumor because of the metabolism of melatonin, which can somehow suppress cancer development.[48] Our result was contradictory from the perspective of functions of melatonin. Long sleepers may be possessed of higher melatonin concentration and subsequently have protective effect in hormone-related cancers.[47]
Numerous studies have confirmed that chronic disease were risky predictors for short or long sleep duration.[49, 50] The U-shaped relationships between chronic diseases and sleep duration has been investigated in several studies,[51, 52] suggesting the importance of sleep health. Furthermore, a U-shaped distribution across short and long sleep duration has also been found for mortality.[53] The mechanism that leads to mortality is not known, and perhaps the co-occurrence of sleep problems (and other problems) may provide clues to pathophysiology.[53]
The large sample size and well representative of Guangdong provincial population were the two major strengths of the study. Multiple complex sampling method was the unique feature of method used in this study. The surveys were conducted by experienced and trained investigators. However, several limitations should be clearly mentioned in this study. First, the study sample only covered the adult population of Guangdong province in China so that the results may not represent the general population of China. Second, participants may misunderstand the definition of actual sleep duration and time of lying in bed. And without objective sleep measurement, the self-reported sleep duration may not represent the real situation. Future studies are warranted to investigate changes to multiple aspects of sleep with objective assessments. Third, the study was designed as a cross-sectional study, and thus the causation of chronic disease and sleep duration could not be examined. Furthermore, other relevant sleep variables were not included into analysis, such as use of hypnotic medication, treatments for specific chronic disease, living conditions, night shift work, cultural factors and so on. These variables may have confounded the results in this study. Another limitation is that we did not exclude other kinds of sleep disorder, such as sleep apnea associated with high BMI or diabetes, restless legs syndrome associated with anemia, sleep walking disorder associated with epilepsy and so on. Other sleep variables, such as sleep quality, daytime functions and sleep efficiency, were not included to measure the sleep condition of participants in this study. In addition, specific information about subtypes of malignant tumor was not collected at the beginning of the study.
Future studies are warranted to investigate changes to multiple aspects of sleep with objective assessments. Further multidimensional studies are also needed to assess the mediating role of mental health in the association between chronic disease and sleep.