Interpretation of findings
The aim of this study was to determine the relationship between different intensity of PA and sleep status among the elderly in Birjand. The findings showed that there is a significant difference in most domains of sleep status between physically active and inactive participants. In other words, active elderly had better sleep quality, sleep satisfaction and sleep efficiency than inactive elderly. These results are consistent with the findings of other studies [33-35]. It seems that many physiological changes during PA (e.g., increased body temperature and melatonin secretion, improved heart rate and immune function) could potentially promote sleep and reduce depressive symptoms .
The results of this study showed that with increasing levels of PA, the quality of sleep increases. This is consistent with results of some studies [24, 25, 33, 34, 36]. These findings support the importance of maintaining daily levels of PA to improve sleep quality in the elderly. On the other hand, our findings were in contrast to the results of Sahin et al. and Wu et al. [5, 37]. This difference may be due to differences in study design, PA and sleep measurement tools, and the lack of control over potential confounders.
Most previous research has focused on the relationship between PA and sleep quality; however, the relationship between PA and sleep duration was examined in this study. Our Findings showed the level of PA was inversely related to duration of sleep. Kredlow et al. in their meta-analysis study, found the effect of exercise on sleep duration in the elderly to be significant but small . The results of our study contradict the findings of some studies that found high PA leads to longer sleep duration [10, 36, 38, 39]. The differences may be due to differences in the type of measurement tools, PA leveling, the average age of participants and differences between place of residence (nursing homes or community). Moreover, as the population of our study were a mixed of rural and urban residents, it is possible that more physically active older adults lived in rural regions and these individuals usually start to gardening at early morning. In order to assess this theory, we compared the level of PA in urban and rural region and we found the elderly lived in rural region were significantly more physically active (data was not showed).
In this study, the elderly who had higher PA were more satisfied with their sleep. This finding is in line with the results of Jacobs et al.' and Park et al.'s studies [40, 41]. However, the hypothesis regarding the association between the demographic variables (e.g., age, gender, and race) and poor satisfaction with sleep was not supported and PA was subcategory of health behavior and had no relationship with sleep satisfaction in Abraham et al. study . Health care providers need to identify poor sleep satisfaction in the elderly and address non-pharmacological methods, especially exercise, to improve sleep because seniors rarely discuss sleep complaints during routine clinic visits, and may incorrectly use over-the-counter (OTC) prescription sleep medications .
The results of this study indicated that the elderly with high PA, had high sleep efficiency which is in line with the result of Kredlow et al.' and Zar et al.' studies [24, 35]. But Murry et al. found that a moderate to vigorous PA and time spent outdoors had a significant effect on predicting total sleep time but had no effect on predicting sleep efficiency. Women may spend more time outdoors in the afternoon and spend more time in bed but have less sleep efficiency . The differences in findings may be due to differences in the mean age of the study population, PA and sleep measurement tools and the effect of outdoor activity time factor on sleep efficiency in Murry study .
According to our findings, it seems that the most influential factor on the relationship between PA and sleep in the elderly was depressed mood. Therefore, it is more likely that by reducing depression, PA has a positive effect on sleep status. As observed in most regression tables, the relationship between PA and sleep characteristics disappeared with the entry of the depressed mood variable into the models. Some studies have shown the positive effects of aerobic exercise on sleep quality and the relationship between reducing depressive symptoms and improving sleep quality . Garfiled et al. found that in the group with elevated depressive symptoms only, physical activity was positively associated with sleep duration in models adjusted for all covariates (age, sex, wealth, ethnicity, smoking, alcohol consumption, BMI, long-standing illness) across low, moderate and high PA groups, in comparison to the sedentary group. Silva et al. and Hsu et al. found a negative effect of depression on sleep quality and PA in the elderly [44, 45]. According to the Diagnostic and Statistical Manual of Mental Disorders, a change in sleep status is one of the criteria for diagnosing depression. A significant number of depressed patients complain of a general deterioration in their sleep. Furthermore, some review studies have shown that PA reduces depression symptoms in the old people . Among the most-accepted explanations for the benefits of physical exercise in treating depression are the increases in the release of neurotransmitters such as serotonin, noradrenaline, and dopamine , as well as endorphins, which promote a general feeling of well-being, relieving depression and producing a feeling of content and of willingness to do things .
Strengths and limitations
Our study has several strengths and limitations. This study is part of the first phase of the BLAS and has a sufficient sample size with representative sample for older population, which allows us to conduct a comprehensive analysis. We examined the relationship between different levels of PA with domains of sleep status using the LAPAQ for the first time in Iranian elderly. We used the valid instruments for evaluation of PA and sleep status among aged people.
The first limitation of this study is that we did not exclude people who were diagnosed with sleep disorders or those taking sleeping medications. The other limitation is the very frail and completely bedridden older adults might not be able to participate in study. Our analysis was limited to cross-sectional data, and we did not examine the longitudinal nature of the relationship between PA and sleep domains.