The 24-hour day is spent engaging in activities that include light-physical activity (LPA), moderate-vigorous physical activity (MVPA), sedentary time (i.e., sitting/lying/reclining posture with energy expenditure <1.5 METs, while awake), and sleep. These behaviors are mutually exclusive and time spent in one behavior affects the time spent in another. The time among these 24-hour behaviors is also associated with cardiometabolic health outcomes, including adiposity. Assessing specific behavioral contexts and their relationship within the 24-hour day is underdeveloped, this includes recreational sedentary screen time (rSST). rSST is sedentary time with televisions, computers, smartphones, tablets, inactive video games, and its relationship with other 24-hour behaviors is underdeveloped. This dissertation works evaluates the relationship between rSST and 24-hour behaviors, and adiposity in adults. The first study reviewed the existing observational and experimental evidence for rSST and its relationship with 24-hour behaviors by conducting a scoping review. From the 75 experimental and observational studies included, the evidence supported an overall positive association between rSST and non-screen sedentary behavior, an overall negative association between rSST with physical activity, and overall positive and negative associations between rSST with various sleep variables. The second study assessed the daily associations between rSST and 24-hour behaviors and how associations are influenced by age, sex, chronotype, and week- or weekend days. The findings include significant negative associations at between- and within-person levels for rSST with non-screen sedentary time, standing, LPA, MVPA, and sleep that were differentially influenced by age, chronotype, and week- or weekend day. The third study examined reallocating time between rSST and 24-hour behaviors and the associations with adiposity (i.e., body mass index, body fat percentage, and waist circumference). The results showed significant associations of replacing non-screen sedentary time with MVPA for both body fat percentage and waist circumference; and no significant associations between rSST and 24-hour behaviors for body mass index. Overall, this dissertation work provides important insights into the relationships between rSST and 24-hour behaviors and their relation to adiposity. These findings can be used to inform future intervention development targeting multiple behavior changes and improving health outcomes.

This thesis paper examines the effects of increased standing and light physical activity in the workplace on postprandial glucose. Sedentary behavior is detrimental to our health, affecting metabolic risk factors. An easy way to implement change is by decreasing sedentary time in workplaces where sitting is common, such as office workspaces. To consider how postprandial glucose is affected by decreasing sedentary time, participants ate a standardized meal for lunch and were asked to decrease their sitting time by replacing it with standing and light physical activity.