This review proposes a biologically plausible working model for the relationship between the 24-hour activity cycle (24-HAC) to cardiovascular disease. The 24-HAC encompasses moderate-vigorous physical activity (MVPA), light physical activity (LPA), sedentary behavior (SB), and sleep. MVPA confers the greatest relative cardioprotective effect - when considering MVPA represents just 2% of the day if physical activity guidelines (30 min/day) are met. While we have well established guidelines for MVPA, those for the remaining activity behaviors are vague. The vague guidelines are attributable to our limited mechanistic understanding of the independent and additive effects of these behaviors on the cardiovascular system. Our proposed biological model places arterial stiffness, a measure of vascular aging, as the key intermediate outcome. Starting with prolonged exposure to SB or static standing, we propose that the reported transient increases in arterial stiffness are driven by a cascade of negative hemodynamic effects following venous pooling. The subsequent autonomic, metabolic, and hormonal changes further impair vascular function. Vascular dysfunction can be offset by using mechanistic-informed interruption strategies and by engaging in protective behaviors throughout the day. Physical activity, especially MVPA, can confer protection by chronically improving endothelial function and associated protective mechanisms. Conversely, poor sleep, especially in duration and quality, negatively effects hormonal, metabolic, autonomic, and hemodynamic variables that can confound the physiological responses to next-day activity behaviors. Our hope is that the proposed biologically plausible working model will assist in furthering our understanding of the effects of these complex, inter-related activity behaviors on the cardiovascular system.
