Walking within nature (Green Exercise) has been shown to immediately enhance mental well-being but less is known about the impact on physiology and longer lasting effects. Heart rate variability (HRV) gives an indication of autonomic control of the heart, in particular vagal activity, with reduced HRV identified as a risk factor for cardiovascular disease. Night-time HRV allows vagal activity to be assessed whilst minimizing confounding influences of physical and mental activity. The aim of this study was to investigate whether a lunchtime walk in nature increases night-time HRV. Participants (n = 13) attended on two occasions to walk a 1.8 km route through a built or a natural environment. Pace was similar between the two walks. HRV was measured during sleep using a RR interval sensor (eMotion sensor) and was assessed at 1-2 h after participants noted that they had fallen asleep. Markers for vagal activity were significantly greater after the walk in nature compared to the built walk. Lunchtime walks in nature-based environments may provide a greater restorative effect as shown by vagal activity than equivalent built walks. Nature walks may improve essential recovery during night-time sleep, potentially enhancing physiological health.

OBJECTIVES:To explore the interrelationship and mediating effect of factors that are beneficial (i.e., antioxidants) and harmful (i.e., inflammation and oxidative stress) to the relationship between sleep and cardiometabolic health.

DESIGN:Cross-sectional data from the 2005-2006 National Health and Nutrition Examination Survey.

SETTING:Nationally representative population sample from the US.

PARTICIPANTS:Age≥ 20 y with sleep data; final analytical sample of N = 2,079.

INTERVENTIONS:N/A.

MEASUREMENTS AND RESULTS:Metabolic syndrome was classified according to the Joint Interim Statement, and sleep duration was categorized as very short, short, adequate, and long sleepers (≤4, 5-6, 7-8, and ≥9 h per night, respectively). The indirect mediation effect was quantified as large (≥ 0.25), moderate (≥ 0.09), modest (≥ 0.01), and weak (<0.01). In general, inflammation was above the current clinical reference range across all sleep duration categories, whereas oxidative stress was elevated among short and very short sleepers. Select sleep duration-cardiometabolic health relationships were mediated by C-reactive protein (CRP),γ-glutamyl transferase (GGT), carotenoids, uric acid, and vitamins C and D, and were moderated by sex. Specifically, moderate-to-large indirect mediation by GGT, carotenoids, uric acid, and vitamin D were found for sleep duration-waist circumference and systolic blood pressure relationships, whereas vitamin C was a moderate mediator of the sleep duration-diastolic blood pressure relationship.

CONCLUSIONS:Several factors related to inflammation, oxidative stress, and antioxidant status were found to lie on the casual pathway of the sleep duration-cardiometabolic health relationship. Further longitudinal studies are needed to confirm our results.