Four core circadian behaviors that improve cardiorespiratory fitness through consistent sleep

To examine the combined impact of four circadian-alignment behaviors-morning sunlight exposure, time-restricted eating, Zone 2 cardiovascular training, and breathwork-on sleep consistency, cardiorespiratory fitness, and parasympathetic activity in adults.

A pre-post quasi-experimental design involving 38 838 healthy adults who participated in the "Core Four Challenge" over 31 days. Participants were monitored using a wrist-worn device (WHOOP Strap 3.0 and 4.0; Boston, MA) tracking sleep and cardiorespiratory metrics. Data were collected across 3 months: baseline, intervention, and postintervention.

Participants significantly increased engagement in all four behaviors compared to matched controls (F ≥ 5610.45, p < .001), leading to improved sleep consistency (F > 11.6, p = .001), resting heart rate (F > 9.2, p ≤ .002), and heart rate variability (F > 6.1, p ≤ .013). Mediation analyses showed that engagement in these four behaviors enhanced cardiorespiratory fitness and parasympathetic activity partially through improved sleep consistency, independent of sleep duration and sleep duration variability.

Increased engagement in the Core Four behaviors was associated with improvements in sleep consistency, which in turn was linked to enhanced cardiorespiratory fitness and parasympathetic activity. This increased engagement persisted for at least 1 month postintervention, suggesting that integrating simple circadian-alignment practices into daily routines may support sustainable improvements in sleep regularity and health outcomes.