With increase in life expectancy, there is growing interest in quality of life. Sleep is receiving increasing attention among the elderly [1]. Recently, the proportion of the elderly population in Korea has increased rapidly, entering an aging society with 16.5% of the population being over the age of 65 years by 2021, according to Statistics Korea. Statistics Korea predicts that the elderly population will continue to increase [1]. According to previous studies, approximately 40% of the elderly complain of some form of sleep discomfort, and there is a report that the prevalence of insomnia in the elderly population in Korea is greater than 50% [2,3].

According to the normal aging process of the body, sleep structure and circadian rhythm change. Therefore, it is necessary to understand the normal sleep physiology of older adults. Elderly people often complain of difficulty falling asleep, which is largely attributed to a decrease in total sleep time [4,5]. Additionally, it is known that the ratio of slow wave sleep (SWS or N3) and rapid eye movement (REM) sleep decreases in the elderly [4,5]. Elderly people often complain that they wake up frequently during sleep, and are always in light sleep. Waking after sleep onset (WASO) generally increases with age [1]. The circadian rhythm of sleep is affected by aging [4,6]. In the elderly, the sleep phase advances, leading to early sleep onset, waking up, and daytime sleepiness [4]. However, changes in normal sleep structure cannot be considered pathological. This suggests that sleep disturbance in the elderly is not an inherent process of aging [7]. Sleep problems in the elderly are affected by several factors, such as various underlying diseases, medications, psychosocial factors, and environmental changes, such as hospital admissions or nursing homes [8,9].

Previous studies have reported a decrease in sleep quality in the elderly, however, no large-scale study evaluating the sleep quality by age group through a nationwide survey has been conducted. By dividing the participants into six age groups, we examined the sleep quality not only of the elderly, but also of all age groups. Therefore, in this study, the sleep quality of various age groups was evaluated using a nationwide survey. In addition, we investigated the differences between those over and under the age of 65 years with respect to the components of sleep quality. Based on the its definition by the World Health Organization, we categorized individuals over 65 years of age as elderly.

This study was conducted with a large sample group obtained from the Community Health Survey, which revealed an association between quality of sleep and age. When adults over 19 years of age were divided into six age groups and PSQI scores were compared, it was confirmed that the average PSQI score increased with age. In addition, when the scores of each PSQI component were compared between the age ≥ 65 years group and the group ≤ 65 years, sleep quality was low in all components except for the ‘daytime dysfunction’ component in the ≥ 65 years group.

In previous studies, a number of research results have been reported that the quality of sleep decreases with age [13,14]. However, no age-related differences have been reported in local studies [15]. In a local study, 208 subjects aged 20–60 years were included. The number of subjects was small, and elderly individuals over the age of 60 years were not included [15]. Therefore, this study overcame the limitations of previous studies involving small groups.

In this study, several factors explained the decrease in subjective sleep quality with increasing age. First, sleep quality may deteriorate due to physiological changes that occur with aging. With normal aging, the total sleep time decreases, and the ratio of slow wave and REM sleep decreases [4,5]. Moreover, the elderly wake up frequently during sleep and fall asleep and wake up earlier than before [1]. These changes may have resulted in higher PSQI scores as people aged.

Second, older adults often have multiple underlying medical conditions that can have a detrimental effect on sleep quality. A previous study found that in the elderly population aged 65 years and older, sleep problems were reported in 36% without comorbidities, whereas the rate of sleep problems with four or more comorbidities increased to 69% [7]. Another study showed that the PSQI score increased as the number of comorbidities increased [16]. In particular, sleep quality was lower in subjects with comorbidities, including psychiatric diseases, cancer, neurological conditions, and angina [16].

Third, drug and substance use can exacerbate sleep problems in older adults. Elderly individuals often take a variety of medications. Some drugs can induce arousal, resulting in changes in the structure of sleep with consequent sleep disturbances [17]. Sleep problems in the elderly are also exacerbated by a decrease in the rate of metabolism, absorption, distribution, and excretion of drugs [17]. Drugs that potentially cause sleep disturbance include antidepressants such as selective serotonin reuptake inhibitors, antihypertensive drugs, bronchodilators, diuretics, beta blockers, and corticosteroids [17,18].

Fourth, psychosocial factors should also be fully considered in sleep disorders in the elderly. After retirement, the elderly have more flexible sleep schedules than before due to disappearance of regular routines. Thus, retirees may have more time to nap or lie down [7]. In addition, social activity also decreases, making many older people feel bored and lonely [7]. Furthermore, different lifestyles change the sleeping habits of older adults. Moreover, older people often experience the death of a spouse or close friend, so they have a period of mourning that sometimes leads to depression, anxiety disorders, and substance abuse [19]. Additionally, the elderly often face the burden of caring for their families or spouses. Consequently, older people often complain of significant levels of stress, which in turn increases the risk of sleep disorders [20,21]. Moreover, the elderly with deterioration of health may experience changes in their sleep environment as a result of admission to a nursing home or hospitalization; these may also affect their sleep [8,9].

In contrast, in the case of daytime dysfunction in the PSQI component, the score was lower in the population over 65 years that in the population under 65 years. It is interesting to note that, unlike with other items, only those aged 65 years and over performed better in this component. This may be because the population aged ≥ 65 years spends less time on work, physical activity, and social activities than the younger and middle-aged populations. Thus, the amount of discomfort arising from daytime sleepiness is small because the underlying daytime activity is insignificant [22].

A higher mean PSQI value was measured in those in their 30s than in those in their 40s, with a difference of 0.11. Although the difference was not large, it is an exception in that the average PSQI score increased as age increased. Similarly, there were more participants in their 30s with PSQI scores higher than 5 than for those in their 40s. In recent years, marriage and childbirth in Korea have been delayed to the early 30s, leading to major changes in lifestyle in the 30s. Sleeping with a spouse and raising young children may change the sleeping environment. In addition, changes caused by forming a new family may act as new stressors in the 30-year-old population, which may affect sleep quality.

A surprising finding in this study was that the percentage of participants with PSQI scores higher than 5 was over 30%, even among young adults under the age of 29 years. This suggests that the importance of sleep problems in young adults should be given attention for appropriate evaluation and treatment.

This study had certain limitations. First, it was a cross-sectional study. Therefore, we cannot prove causality that age is the cause of worsening sleep quality. In addition, the survey results may be inaccurate, as the person interviewing the participant was not a sleep-knowledgeable medical professional. Third, in the community health survey, the average sleep time was based on the respondent’s memory. Therefore, there was a high possibility of recall bias.

Nevertheless, this study had several strengths. First, it is a large community-based study that reflects the trend of the entire population, unlike previous studies with small groups. Additionally, unlike previous studies that did not involve different age groups in the analysis, this study carried out a detailed analysis to uncover the influence of increasing age and showed a correlation between age and sleep quality.