This study adopted a comparative case study design to examine the differences between the characteristics of body composition, sleep quality, and autonomic activity in active older adults who were younger than their chronological age. This study set the following inclusion criteria: active older people aged 65 years and older who worked in the hospital. Despite the retirement age being 65 years, these participants worked at the hospital in a capacity similar to the working generation (18 - 64 years old). Since some working older adults were not in good health, it was necessary to collect many older adults in this study.

The two cases included in the analysis were those who met the criteria based on data collected from 48 participants. Their activities and daily living abilities were verified by the Japan Science and Technology Agency’s ability index (JST-IC) and instrumental activities of daily living (IADL) [22] [23] . This included those who had no identified health problems.

Meanwhile, the exclusion criteria were as follows: those 1) diagnosed with dementia, diabetes, or cancer, 2) severe heart disease or fragile skin conditions, or 3) unable to answer the questions on sleep and lifestyle or did not agree to provide informed consent.

This study selected participants who had good sleep quality according to the Japanese version of the Pittsburgh Sleep Quality Index (PSQI-J), good heart rate variability (HRV) balance which indicated autonomic nervous system activity, good body composition (muscle and body fat mass, grip strength, and sarcopenia assessment), and were younger and fitter than their actual age. For the control cases, age-appropriate older adults were matched by age and gender. These were older men and women with poor sleep quality and imbalanced autonomic nervous system activity whose body age was the same as their actual age.

Figure 1 illustrates the case selection process.

Data were analyzed using IBM SPSS Statistics for Windows version 29. Welch’s analysis of variance (ANOVA) with post-hoc tests (Games-Howell test) were conducted for each case. Significance was set at p < 0.01.

The study was approved by the Ethics Committee of Tokushima University Hospital (#3046), Mifune Hospital Clinical Research Ethics Review Committee, where the study was conducted (#20180502), and the Ethics Committee of Kochi University Hospital (ERB-107865). Informed consent was obtained from the participants before the study began. All participants were informed regarding the purpose of the study and methods used. This study reassured them that their personal information would be protected and kept in a secure location in the researchers’ office under lock and key. In addition, the results were reported in aggregate form. The participants were informed that the data generated would be used only for research purposes.

This study selected two cases from the 48 participants who had a metabolic age younger than their actual age, good sleep status, and balanced autonomic nervous system activity. These two cases had good sleep quality, low PSQI scores, no sarcopenia, strong muscle and grip strength, balanced autonomic nervous system activity, and were considered to be much younger than their actual age.

The control cases (metabolic age was age-appropriate), had poor sleep quality, unbalanced autonomic nervous system activity, and physical age close to their actual age. Despite the usual retirement age being 65 years, the four cases worked at the hospital in a capacity similar to the working generation (18 - 64 years old). Table 1 shows their basic attributes and measurement values/results.

The four cases in our study were as active as the working generation. Their scores for strength, assistance with walking, rising from a chair, stair climbing, falls (SARC-F), and grip strength showed that they did not have sarcopenia. Poor-quality sleep in older adults has been related to functional limitations, such as a reduction in hand grip strength [37] and physical activity [38] , and muscle mass and function [39] .

Grip strength is thought to be a factor that reveals sleep disorders [40] and problems related to sleep quality and duration [37] [41] . The grip strength of Cases A and B, along with their favorable sleep status, as indicated by their PSQI, appeared to confirm these past studies [37] .

Cases A and B had lower percentages of body fat, and their metabolic (physiological age) [32] was 15 years younger than their actual age. Case A had a Muscle-Q score that was standard for women of approximately the same age. Case B’s Muscle-Q score was higher than that for men of approximately the same age. Men tend to have significantly greater muscle mass than women in all parts of the body, regardless of age [42] . Case B, a man, was believed to have a larger muscle mass partly due to the influence of total and free testosterone [43] .

It has been reported that weight and visceral fat continue to decrease linearly with aerobic exercise [44] ; this would explain why older individuals who engaged in extensive physical activity were able to keep themselves in the same physical shape as young people. According to this data, it was suggested that they were able to maintain good physical strength and agility, similar to that of younger adults who had a high metabolic rate, by maintaining a high level of physical activity.

Characteristics of Cases A and B showed lower percentages of body- and visceral fat, and higher body water. For older adults of similar muscle mass, those with higher intracellular water (ICW) had higher functional performance and lower risk of frailty, which suggested that cell hydration had a protective effect independent of muscle mass [45] . However, it is unclear whether the observed positive effect of high ICW content was due to greater muscle mass or better muscle quality and cell hydration [46] .

Body water percentage tends to be higher in men than in women [47] and decreases with age [46] . It also tends to be lower and higher in those with a high and low body fat percentage, respectively [46] . The body water percentage of people whose body fat percentage was within the proper range was approximately 55% - 65% for men and 45% - 60% for women [31] . The body water percentage of Cases A and B were within the appropriate range.

Cases C and D also had high grip strengths, and neither had sarcopenia. Owing to their activity levels that were comparable to the working generation, their Muscle-Q scores were assessed as being standard values for people of their generation. However, their sleep quality was poor. Researchers have reported that in ordinary aging, sleep debt had a negative impact on carbohydrate metabolism and endocrine function, which suggested that a lack of sleep aggravated age-related chronic diseases [48] . Even older people who have few comorbidities and lead independent lives experience decreasing sleep quality (PSQI) as age increases.

Cases C and D had high body fat, and their physiological age was close to their actual age. Cases C (female) and D (male) had poor-quality sleep. Women have a high risk of metabolic syndrome, regardless of their sleep duration, whereas in men, the shorter the sleep duration, the higher their risk [49] . Sleep duration of five hours or less also affects the distribution of centrosome fat and causes an increase in body fat percentage [50] . The process by which lack of sleep leads to obesity may involve changes in the neuroendocrine system and metabolism, as well as behaviors/activities engaged in while awake [51] . Although weight and body fat may be constant, the distribution of fat changes with age. In addition, there is a higher risk of it shifting to the abdominal area [32] . Therefore, attention should be paid to buildup of visceral fat often attributable to age-related decreases in sleep quality, aside from the body fat percentage.

Regarding the HR-mean, Cases A, C, and D had high pulse rates, even when resting prior to exercising. Given that older adults generally engage in less physical activity and have a slower metabolism than adults, their pulse rate is reportedly 10 to 20 pulses per minute lower than that in adults. Hence, the heart rates of these three cases may have risen owing to psychological tension, such as dealing with unfamiliar researchers whom they had not met before, being placed in the unfamiliar setting of a conference room, and undergoing examinations [52] .

The cases’ pulses were significantly lower after exercise than beforehand. However, the pulses were not significantly higher during exercise than beforehand. Case A, a woman, had a significantly higher HR before exercise than after. Her sympathetic nerve activity was significantly higher during and after exercise than beforehand—it did not decline even after exercise. Case B, a man, showed lower sympathetic nerve activity during and after exercise than beforehand. Case C, a woman, showed no changes in her sympathetic nerve activity before, during, or after exercise.

During exercise, cardiovascular parameters change to supply oxygen to the working muscles and preserve the perfusion of vital organs. At the onset of exercise, heart rate (and cardiac output) elevation is mostly mediated by central command signals via vagal withdrawal. As work intensity increases and the heart rate approaches 100 beats/min, sympathetic activity begins to rise, further increasing the heart rate and plasma norepinephrine concentration and vasoconstricting vessels in visceral organs. With exercise cessation, the loss of central command, baroreflex activation, and other mechanisms contribute to a rise in parasympathetic activity, causing a decrease in heart rate despite maintained sympathetic activation [53] .

Post-exercise exponential decline of the heart rate is an intrinsic property of intact circulation and is independent of autonomic control. The heart rate rapidly decreases during the first 1 - 2 min after exercise cessation, and then gradually thereafter. During recovery from moderate and heavy exercise, the heart rate remains elevated above the pre-exercise level for a relatively long period (up to 60 min). The increase in parasympathetic activity causing heart rate deceleration after exercise is independent of basal parasympathetic tone to a large extent [54] .

The action of the sympathetic nerves does not change significantly even with age. Meanwhile, the action of the parasympathetic nervous system reportedly declines with age, making people likely to fall into a state of autonomic imbalance, with only the sympathetic nerves functioning strongly [55] . Moreover, during mild exercise, if the load is too light, sympathetic nervous activity cannot be sufficiently controlled [56] .

In our study, however, Case A experienced no reduction in sympathetic nervous activity during or after exercise. Furthermore, Case C saw no changes attributable to exercise, with the sympathetic nerve activity predominating; that is, no switchover to the parasympathetic nervous system had taken place. Research has noted that the higher a person’s aerobic exercise ability, the swifter their post-exercise HRV recovery [57] . Our participants were extremely active in their everyday lives and exerted themselves to a level comparable to that of the working generation. Thus, they were physically young and it influenced the switchover to autonomic nervous activity.

Regular physical activity has positive effects on cardiovagal nerve activity and decreases the influence of aging on the autonomic nerve control of the heart rate [58] . Our participants’ average pulse was significantly lower after exercise than beforehand. However, since sympathetic nervous activity in Case D was greater beforehand than during exercise, pre-exercise psychological stress appeared to have decreased with exercise.

A balanced diet and high sleep quality are known to be important in regulating the autonomic nervous system. Habitual physical exercise reportedly reduces the risk of lifestyle diseases and deterioration in life functions. Furthermore, it is useful for maintaining and enhancing endurance and muscular strength [59] , and can help reduce locomotive syndrome and mild cognitive disorder [60] . Meanwhile, since light-intensity physical activity is effective in improving the subjective sleep quality of older adults, while moderate to vigorous physical activity has the opposite effect [57] , continued light exercise may be effective in improving sleep quality. Raising the intensity of existing physical activity at work, in daily life, or during regular leisure activities appears to be useful in preventing premature death [61] . To increase healthy life expectancy, people should be encouraged to increase their level of physical activity, such as exercise and work, as well as to regulate their meals and sleep and maintain a low level of autonomic nervous system activity.

This comparative case study compared two pairs of older people with opposite muscle mass, basal metabolic rate, and sleep status. The results demonstrated that older adults with more muscle mass and higher basal metabolism were younger than their actual age, and had better sleep status and a good balance of autonomic nervous activity. These findings can be used to notify healthcare professionals of the importance of promoting physical activity among older adults.

However, this comparative case was a small-scale study conducted to inform, predict, and direct an intended future full-scale study. It revealed that older adults with more muscle mass and higher basal metabolic rate were younger than their actual age and had better sleep status. Large-scale studies should be conducted to clarify the living conditions including dietary habits and lifestyles of older adults with the aforementioned health conditions.