In this study, we first characterized physical activity-counting patterns by examining the temporal shape of physical activity that reflects how an individual's activity is distributed throughout 24 hours. Then, we defined physical activity-count behaviors by identifying the dominant physical activity-counting patterns, showing the repetitive features of the physical activity that individuals engage in throughout the week. This procedure was achieved using unsupervised ML techniques. The current study identified five physical activity-counting patterns and four physical activity-count behaviors. Based on the identified physical activity-count behaviors, we revealed that individuals exhibiting E dominant (Evening dominant) behavior had a higher prevalence of depressive symptoms compared to those with M dominant (Morning dominant) behavior, irrespective of total activity count. This association was confirmed after adjusting for potential confounding factors. We also found that individuals with a higher prevalence of depressive symptoms have higher day-to-day variability.
Recently, studies using ML to analyze a huge data set of physical movements appeared with potential applications in epidemiology, public health, and human behavioral science [33–36]. The ML is applied to identify or categorize behaviors such as sleeping, lying, sitting, standing, walking, ascending or stairs, running, and other routine activities like biking, household chores, and yoga [37]. However, few studies have directly extracted or categorized temporal physical activity patterns by ML but could not differentiate between the temporal physical activity pattern and the intensity [50] or the total amount of physical activity [51]. Our study focused on categorizing each 24-hour activity-count into several clusters directly and identifying the behavioral tendencies of each individual based on the probability of dominant activity patterns. Our approach based on the data-driven analysis could be more straightforward, insightful, and beneficial to handle temporal patterns and dominant behaviors of human physical activity directly, although a relatively large complete data set is required.
Previous studies showed various results regarding the association between depressive symptoms and physical activity patterns, such as the timing of activity [27, 28], consistent with our result that a specific temporal behavioral pattern (E-dominant) is associated with depressive symptoms. In these reports, the higher depression severity was linked to increased activity late at night in the calculated 24-hour activity pattern [27] and the depressed participants had significantly lower physical activity counts during the daytime compared to healthy controls [28]. In contrast, the other observational and case-control studies have indicated no association between depressive symptoms and activity timing [29, 30]. Different classifications of the activity patterns due to the distinct procedures might result in inconsistent conclusions since these observational and case-control studies did not observe or detect the evening or nighttime activity patterns, possibly leading to no association of physical activity patterns with depressive symptoms.
In our results, the E pattern showed the highest activity between 6 p.m. and 9 p.m., and the E dominant behavior showed a higher prevalence of depressive symptoms. The association between E dominant behavior and the depressive symptom might reflect a disruption of circadian rhythm, which could play a crucial role in regulating mood and sleep-wake cycles since disruptions in the circadian rhythms, such as delayed sleep-wake timing, have been associated with increased risk of depressive symptoms and mood disorders [52]. Alternatively, individuals with depression may reduce daytime activity and compensate for total physical activity by increasing their evening activity when they feel energetic [53, 54]. The latter possibility might explain our result in which E dominant (risk group) and M dominant (control group) behavior showed a similar total count activity.
Many previous studies reported that there is a connection between depressive symptoms and low physical activity levels [21, 28, 29]. These findings suggested that total physical activity would be significant for the association with depressive symptoms. For instance, individuals with depression had lower 24-hour physical activity levels compared to controls without depression, and those with acute depression exhibited a marginal significance towards later timing of daily activity peaks, particularly in the evening [21]. Similarly, individuals with depression displayed significantly lower physical activity levels between 7 a.m. and 10 p.m. compared to healthy controls, considering both timing and activity levels [28]. Additionally, the presence and severity of depressive and anxiety disorders were associated with reduced overall daily activity levels, but no significant association was found with the timing of activity [29]. In our study, E-dominant behavior exhibited a total activity count similar to AD + M-dominant behavior (Table 1). However, only E-dominant behavior was found to be associated with depressive symptoms compared to M-dominant behavior (Table 2). On the other hand, the AD dominant behavior has a higher amount of activity count compared to the M dominant behavior (Table 1), but the AD dominant behavior is not associated with depressive symptoms (Table 2). These results suggest that the physical activity behavior based on the dominant physical activity pattern might be a more significant parameter than the total amount of physical activity to explain the association with depressive symptoms in a particular procedure.
We also examined the day-to-day variability of activity patterns within different activity behaviors (Supplementary Table S3) and their relationship with depressive symptoms. We found that AD dominant, M dominant, and AD + M dominant behaviors exhibit the least day-to-day variability, with nearly 80% of their dominant activity patterns corresponding to their respective behaviors. However, the E dominant behavior shows considerably more variability, as only 49% of E patterns align with E dominant behavior, while the rest are distributed among other patterns (Supplementary Table S3). In this analysis, we observed that individuals with higher day-to-day variability (E dominant behavior) had a higher prevalence of depressive symptoms compared to those with less day-to-day variability in their activity behavior (AD dominant, M dominant, and AD + M dominant behavior) (Table 1). These findings indicate that the variability and temporal shape of activity patterns may play a significant role in the relationship between physical activity and depressive symptoms. This underscores the importance of considering these factors when seeking to understand the connection between physical activity and depressive symptoms.
Recent research has yielded valuable insights into the multifaceted influences on depressive symptoms across different life stages and factors, such as age, BMI, gender, and employment. Age was associated with a gradual increase in depressive symptoms, especially among older individuals [55]. Furthermore, a study on the relationship between depressive symptoms and BMI anticipated that overweight or obese individuals would experience more severe depressive symptoms, regardless of their racial background [56]. There are gender differences in depressive disorder, where women are more likely to have a higher prevalence of depression than men [57]. Unemployment could be associated with the onset of clinical major depression [58]. Our study uncovered a higher prevalence of depressive symptoms among individuals engaged in E-dominant behavior compared to those with M-dominant behavior, even after adjusting for confounding factors above. Our finding provides insight into the contribution of the physical activity pattern to understanding depressive syndrome, emphasizing the importance of individualized assessment of physical activity behavior to understand depressive symptoms.
The extent to which our machine-learning procedure could be generalized remains in future studies. However, our approach in this study could be applicable to other diseases or disorders in which physical activity could be closely related to the diagnosis. Temporal physical activity patterns might also indicate a meaningful link to cardiovascular disease (CVD) and obesity [50, 51]. Clustering temporal physical activity patterns by machine learning could enable comprehensive description or analysis at a higher resolution.
A major limitation of the current study is that the causal relationship between activity-count behavior and depressive symptoms could not be established because of the cross-sectional design. In addition, the assessment of depressive symptoms was used by self-reported questionnaires. Although PHQ-9 used in this study has high sensitivity and specificity [45–47], we did not precisely diagnose the depression. However, our study would provide a possible impact of physical activity patterns rather than the total amount of physical activity on the relationship with depressive symptoms. Evaluating the physical activity/behavioral patterns, such as temporal changes or probability, may lead to valuable insights into the association between physical activity and depressive symptoms. Our findings also have a potential benefit to developing software for wearable devices alerting or reminding a risk to prevent the progression of depressive symptoms. These findings might have implications for improving the management and prevention of depressive symptoms in clinical and community settings.