As estimated, the number of older adults aged 65 years or above was approximately 771 million in 2020. This number is expected to increase to 1.6 billion, which accounts for 16% of the world's population by 2050.1 With the population aging, sleep disorders of older adults have attracted more and more research interests. Sleep disorders are one of the common health issues among older adults.2 It was reported that the prevalence of sleep disorders among older adults varied from 10.4% to 62.1%.3,4 Sleep disorders including advanced sleep timing (i.e., earlier bedtimes and rise time), decrease in sleep quantity, longer sleep-onset latency (i.e., longer time taken to fall asleep), more awakenings, more fragile sleep, and less deep sleep are increasing gradually with age.5 Sleep quality is an important indicator to assess whether some sleep disorder is present. Poor sleep quality is a common characteristic associated with all kinds of sleep disorders.6 It is estimated that, approximately 40% to 70% of older populations have poor sleep quality.7 Poor sleep quality is associated with increased mortality and morbidity in older adults.8 Older people with poor sleep quality are found to have an increased risk of cardiovascular disease-related death, dementia, depression, anxiety, and frailty.3,9, 10, 11, 12 Because of these adverse effects on physical and mental health of older adults, the importance of sleep quality improvement has been increasingly recognized.13,14

Multiple factors can be associated with a decline in sleep quality in older adults, including chronic health conditions, mental disorders, and social factors.3,15,16 Loneliness is frequently reported and documented as one of the social factors that associated with poor sleep quality.16,17 Loneliness can be categorized into emotional and social loneliness.18 The lack of intimate relationships, that is characterized by a small number of connections, is the primary factor related to emotional loneliness.19 Furthermore, shifting social responsibilities, deteriorating health conditions, and declining mobility are also associated with a higher risk of emotional loneliness in older adults.20,21 Social loneliness is described as a lack of participation in social networks such as family, friends, and neighbors.22 As older adults age, they have fewer social relationships and social situations, which can lead to increased loneliness.23,24 A growing number of studies have shown that a higher level of loneliness predicts poorer sleep quality over time in older adults.16,25, 26, 27 From a medical perspective, loneliness can lower immune system function and raise neuroendocrine dysregulation, both of which can have a detrimental impact on sleep quality.28,29 However, some studies found that loneliness has no concrete association with sleep quality after controlling anxiety, depression, stress, social support, age, gender, and other variables.27,30 These inconsistent findings made it challenging to develop targeted interventions on sleep quality improvement for older adults. Thus, it is essential to clarify the association between loneliness and sleep quality to fully inform intervention development in the future.

So far, there have been two systematic reviews on the association between loneliness and sleep.31,32 Nevertheless, the two systematic reviews were conducted in the whole population, instead of the older population in particular. Older adults have their unique characteristics regarding sleep patterns and loneliness. As known, sleep patterns change with aging, such as decreased total sleep efficiency and deep sleep, and increased number of nocturnal awakenings and time spent awake during the night.5,8 Older adults are more prone to experience poor sleep quality than younger people. On the other hand, older adults are more likely to experience loneliness due to bereavement, the death of a spouse/partner, reduced socioeconomic status after retirement, physical impairment, and living alone.26,28,33 A specific systematic review and meta-analysis are accordingly necessary to investigate the association between loneliness and sleep quality in older populations. Therefore, the current systematic review and meta-analysis aimed to review studies and explore the association between loneliness and sleep quality in older adults.