The main objective of this study was to evaluate the association between diseases and sleep duration in older adults who exercise regularly. The results showed that older adults with hypothyroidism and fibromyalgia were more likely to have short sleep duration, i.e., to sleep less than seven hours per night. As a secondary result, we also found that participants with osteoarthritis, depression, diabetes, heart disease, fibromyalgia, and urinary incontinence had a higher global PSQI score, indicating poor sleep quality.
The scientific community has made efforts to understand the relationship between sleep duration and health outcomes, and there is a particular need to analyze this association in the elderly population . Sleep physiology undergoes significant changes throughout life and sleep duration distribution varies with age . Within this context, both short and long sleep durations appear as “villains” of health in older adults [12, 13] and are associated with different diseases and adverse conditions [14–17].
We found that older adults with hypothyroidism are more likely to sleep less than seven hours per night. Hypothyroidism is a condition caused by dysregulation of thyroid hormone production. Its symptoms include drowsiness, fatigue, lethargy, and intolerance to cold, which vary according to age and gender . Our result is in line with a population-based study in which adults with subclinical hypothyroidism were more likely to have short sleep duration (OR=1.15) regardless of sex or age ; the conditions was also associated with increased sleep latency and a higher frequency of sleep disturbances. However, another study involving 4,945 adults showed that only long sleep duration was associated with a risk of hypothyroidism .
The mechanisms underlying this association have not been well established. However, some hypotheses can be raised. Studies have shown an interaction between sleep and the immune system [32, 33]; the latter is also associated with hypothyroidism since the disease is more prevalent in patients with autoimmune conditions such as type I diabetes . In this respect, the relationship between sleep duration and hypothyroidism occurs indirectly, mediated by the immune system. Another hypothesis is based on the inhibitory effect of sleep on the secretion of thyroid-stimulating hormone (TSH)  in which poor sleep, including short sleep duration, can affect the hypothalamic-pituitary-thyroid axis, increasing TSH levels .
Another finding is the higher probability of short sleep duration in older adults with fibromyalgia when compared to those without the disease, regardless of gender and education. Fibromyalgia affects approximately 2.7% of the world population  and is characterized by a group of symptoms that include fatigue, pain, memory problems, and sleep disturbances . Sleep problems are common among people with fibromyalgia, with nocturnal restlessness, involuntary leg movements, frequent awakenings, and the perception that sleep is light and not restorative being common complaints .
The presence of fibromyalgia actually affects sleep. In a meta-analysis, Wu et al.  found that individuals with fibromyalgia have a shorter sleep duration, in addition to poor sleep quality. A classic study also identified a relationship of poor sleep quality and short sleep duration with fibromyalgia . This association seems to be bidirectional since the authors concluded that a night of nonrestorative sleep results in significantly more pain on the following day , which is the main complaint of patients with fibromyalgia .
Fibromyalgia disrupts sleep in a vicious cycle that primarily involves the autonomic nervous system; pain caused by the disease increases cardiovascular sympathetic activation and reduces sleep efficiency, resulting in lighter sleep and more awakenings during the night, events that lead to abnormal cardiovascular neural control and heightened sensitivity to pain . In addition, slow-wave sleep (SWS) – one of the sleep phases – is reduced in fibromyalgia patients compared to healthy individuals . The duration of SWS is regulated homeostatically in such a way that it is directly related to the duration of wakefulness , which increases SWS when prolonged and decreases it when reduced . Thus, the decrease of SWS in fibromyalgia has been suggested to be an indication of the impairment of the homeostasis, directly affecting sleep duration .
It is important to remember that the older adults studied here are exercise practitioners. Exercise has been reported as an important non-pharmacological intervention in the treatment and prevention of numerous diseases [44, 45]. In patients with hypothyroidism, exercise can improve endothelial function, VO2max, lipid profile , serum TSH levels , and quality of life . For patients with fibromyalgia, exercise has positive effects on fatigue [49, 50], pain [50, 51], sleep quality [49, 52], quality of life [50, 51], and physical function .
Exercise is also independently associated with sleep in older adults , improving sleep quality, latency, and duration [54–56]. Although the literature shows that physically active older adults have a lower prevalence of sleep problems than their sedentary counterparts [22, 57], the participants in this study exhibited a significant prevalence of poor sleep quality (47.7%) and short sleep duration (43.1%). This result reinforces the multifactorial origin of sleep complaints, involving not only behavioral factors such as physical exercise but also psychological, environmental and pharmacological factors .
This study has some limitations such as the self-reported diagnosis of the diseases by the older adults and the use of indirect measures to assess the quality and duration of sleep. However, the applied questionnaire (PSQI) is widely used in the literature and is a validated and reliable instrument. In addition, it was not possible to indicate a causal relationship between the variables since cross-sectional studies do not allow to establish causality. Thus, longitudinal studies are necessary to better monitor changes in sleep duration and quality during the health-disease process. On the other hand, the study involved a significant sample and is part of a university extension program, portraying more reliably the reality of older adults. Furthermore, this is the first study to link sleep duration to the presence of disease in older adults who exercise regularly.