Exposure to nature has wide ranging positive effects on health, particularly mental wellbeing in comparison to indoor environments [1
] or synthetic or built environments [2
]. Exercising within nature (termed Green Exercise) is psychologically restorative, immediately enhancing mood and self-esteem [3
]. A handful of studies have investigated the potential of Green Exercise to promote immediate physiologically restorative effects [6
] with fewer assessing longer-lasting effects [2
Compared to equivalent non-Green Exercise conditions (either viewing scenes of built environments on a screen whilst exercising on an ergometer or performing physical activity in an indoor or “built” outdoor environment), Green Exercise participation promotes post-exercise decreases in heart rate (HR) [8
], systolic (SBP) [6
] and diastolic blood pressure (DBP) [6
]. Light intensity walking in a natural environment enhances physiological markers indicative of recovery from stress [7
], possibly due to blunted responses in catecholamines and cortisol [9
]. Interestingly, alterations in autonomic nervous system control (ANS) (indirectly measured using heart rate variability (HRV)), have also been reported. HRV is a well-established non-invasive tool giving an indication of the changes in vagal and sympathetic control of the heart, by providing a measurement of inter-beat differences in HR that takes into account both the parasympathetic and sympathetic contributions to the sino-atrial node regulation of HR [12
The ANS plays an important role in the maintenance of homeostasis and in modulating responses to stressors [10
]. Green Exercise has been shown to enhance cardiac vagal activity (parasympathetic activity). To date most of the effects of Green Exercise have been measured either during or immediately after exercise; which may mask benefits of green due to the effects of exercise [13
], particularly when exploring the ANS which will be directly affected by the exercise. When measures are taken one hour or more following Green Exercise, even in a controlled environment, additional physical or psychological and sensory inputs that are not related to the Green Exercise (or control activity) may alter physiology markers. The longer-term physiologically restorative properties of Green Exercise remain unexplored.
Sleep is a behavior that promotes bodily restoration and is characterized by circadian variation in metabolic, endocrine and autonomic systems. HRV monitoring during sleep has identified a nocturnal zenith in cardiac vagal activity coincident with peak melatonin and a nadir in circulating cortisol concentrations [14
]. Augmenting vagal activity during sleep could enhance restoration and sleep quality.
There is evidence to suggest a relationship between exercise and sleep quality [15
], but to date, the majority of research has been reliant on self-report methodologies. Two hours of forest walking increased duration of sleep and self-reported sleep quality [18
]. Considered together with the outlined previous research findings, which forward that nature environments promote physiological restoration, it seems possible that Green Exercise participation might maximize sleep-related physiological restoration associated with time spent in nature. Despite the outlined current knowledge linking physiological restoration, environmental exposure, physical activity and sleep; research is yet to examine the impact of performing physical activity within different environments on autonomic function during sleep. The current study tested the hypothesis that lunchtime walking in nature increases nocturnal cardiac parasympathetic activity compared with a built walk.
The current study is the first to explore alterations in the ANS during the early part of a night sleep following a bout of Green Exercise at lunchtime. The current study was exploratory and extends the current literature by examining the longer-lasting health enhancing effects of Green Exercise. It is the first study to compare a short walk in a Green environment to walking in a built environment on night-time HRV measures. Perhaps surprisingly, the hypothesis was supported; compared to a built walk, a short Green walk at lunchtime resulted in significantly augmented parasympathetic activity during sleep. That is, both the overall HRV measure (SDNN) and the more specific parasympathetic measures (rMSSD and SD1) were significantly greater in night-time recordings following the Green walk. This may suggest that individuals following a Green walk experience greater restoration during this period of sleep. However, the current study did not extend duration of sleep following a Green walk. This finding is in contrast to previous work that identified greater duration of sleep after a 2 h forest walk in comparison to a night with no walking during the previous day [18
]. The previous study compared walking in contrast to no walking and the exercise itself may have caused the enhanced sleep duration. The short walk in the present study may have had a smaller effect than a longer walk i.e
., it has a smaller stimulus. The intention of the current study, however, was to explore whether walking during lunchtime in a workplace setting altered night-time vagal activity. A two hour walk would not be plausible in this setting.
As human beings have lived in the natural environment for most of their existence (over 5 million years), it is likely that the physiology of humans is best suited for natural environments and these are likely to promote relaxation and restoration. Previously reported physiological measures obtained during time spent either in natural or simulated environments with or without exercise have provided evidence to support this argument [6
]. The results of the current study are consistent with previously reported findings that adrenaline, noradrenaline and BP remain reduced in the evening following a daytime walk in a forest [8
]. As the parasympathetic nervous system is dominant in restoration [10
], the current findings support the notion that compared to equivalent exercise in other environmental settings, Green Exercise promotes physiological restoration and health. Immediate and short-term similar physiological benefits of Green Exercise have been previously reported [8
], but this is the first study to suggest that effects on ANS control, and in particular on parasympathetic activity, are longer lasting and might promote physiological restoration during sleep [30
]. Enhanced vagal activity at night-time is necessary for restoration and is essential for health. High vagal activity during sleep is part of the normal circadian variation and is observed in healthy individuals, decreases with age [31
] and is reduced in cardiovascular disease [32
]. The potential capacity of Green Exercise for elevating nocturnal vagal activity will be useful to explore further. Interestingly, in the current study mean RR interval was not different between the two walks. This may be in part that RR interval was already high during sleep (having a ceiling effect). Additionally, there will be an increase in vagal neuron excitability occurred followed by a rebound inhibition of these vagal neurons by inspiratory drive, leading to augmented HRV values, but with an unchanged mean RR interval (i.e
., there is greater fluctuation around the mean (as seen by SDNN) but the mean does not change).
To date studies that have explored physiological responses following Green Exercise, even in a controlled environment, have found it difficult to compare changes in physiology especially when collecting data greater than one hour following exercise. It is likely that the physiological changes that may occur as a result of being exposed to different environments are masked by the physiological effects of the exercise itself [13
], and this is particularly important when exploring the ANS via HRV. Measuring during sleep might be advantageous as individuals are likely to be experiencing minimal physical disturbances (inactivity of voluntary muscles) and psychological inputs from external sources, as at this time consciousness is in a reduced or absent state, and sensory activity is limited. It may, however, be useful in future to explore HRV in the one hour following the walks within a controlled environment to allow comparisons to be made to overnight recordings.
A strength of the current study was its cross-over, within subject design, with participants acting as their own control by completing both walks (Green and Built). This ensured that the between subject variability prevalent in the HRV data did not confound the statistical analysis, thus facilitating greater statistical power for the given sample size. Additionally, there were no differences in cardiovascular measures between testing days confirming that all participants were starting with a similar baseline on both occasions. We carefully controlled for duration and intensity of the walk. This was confirmed by mean HR during walks, change in HR from baseline and RPE during both Green and Built walks were comparable. Temperature during both Green and Built walks was also similar. It is therefore unsurprising that there was minimal effect of which order the walks were completed in.
The duration of the walks in the current study (approximately 17 min) was comparatively shorter than the 2 h used in a previous study of this kind [18
]. This indicates that as well as longer walks during leisure time, short walks during lunchtimes, perhaps at workplaces, may promote restorative benefits beyond the immediate future. This suggests that a shorter walk, which fits into a working day, may induce longer lasting physiological benefits. Previous work has shown that as little as five minutes can alter short-term changes in mood [3
]. Further investigation is required to identify the optimum dose (duration and intensity) of Green Exercise for enhanced physiological outcomes during sleep.
HRV data was collected for a period 60–120 min from sleep onset (as noted in diary). Although this time period was selected to reflect the minimal sensory input and muscle activity, it is possible that the data analyzed included some data representative of rapid eye movement sleep. However, this is unlikely due to minimal artifacts in ECG, stationarity and almost identical RR intervals for both walks.
There are several further limitations to the current study. The first is the small sample size. However, the p values and effect sizes suggest that there appears to be some interesting alterations in HRV which should be explored further. Although the participants completed the walks in a random order, those that completed the Green walk first were significantly older as a group than those who undertook the Built walk. It may be that this may have influenced the results but no order effect for physiological measures was identified. Another potential limitation is that the research assistant consciously or unconsciously may have encouraged more positivity during the Green walk. This is true of any walks where participants are guided. In the design of the current study it was considered whether participants could walk without a guide but this was not deemed plausible as the walks that were used had many changes in direction and without a guide the participant may have become lost or walked an alternative route. Additionally, in future it would be useful to compare HRV recordings following the walk with overnight recordings. However, HRV needs to be collected in a controlled environment, and it is possible that immediately following walks, any effects of environmental settings on HRV may still be masked by effects of the recently performed exercise. That is, environmental influences on HRV may only become measureable when the masking influences of exercise have subsided.
The aim of this preliminary study was to investigate ANS activity during normal habitual sleep and record physiological data with minimal disturbance to the participant i.e., within the home setting. As participants remained in their regular and home settings following the walk, it ensured good ecological validity of the demonstrated findings. Allowing the participant, however, to undertake normal habitual activities within their normal settings may potentially affect the physiological outcomes several hours later, including during sleep. To try to eliminate this participants were asked to complete a diary and were asked to keep the second day as similar to the first day as much possible. In the current exploratory study, the alternative of asking participants to stay within a controlled environment was not plausible. Controlling the environment following the walk, including the sleep environment, should be considered for future work.
Additionally, other relevant physiological measures including actigraphy and/or EEG should be examined in future work. Actigraphy has been used previously [18
] before and after forest walking and can give an indication of sleep duration and activity during sleep. Combining HRV and actigraphy may therefore be very useful and give a more accurate indication of sleep time. If a more detailed sleep analysis is required by obtaining the EEG, the participant would need to be placed in a controlled sleeping environment, but this may affect their habitual sleep patterns, influencing HRV measures. Both actigraphy and EEG will allow a more thorough investigation of whether the sleep becomes more restorative by altering the duration of slow wave sleep, rather than altering the duration of sleep for the whole night i.e
., the quality of sleep increases. This might have important implications to enhance the restorative part of sleep or help those that have sleep disorders. Most sleep disorders have modifications in the physiology of sleep, which manifests itself in signs and symptoms of sleep loss and quality, which are shown to be independent risk factors for cardiovascular morbidity and mortality [33
]. Therefore ANS cardiovascular control could be a potential physio-pathological link between sleep disorders and their physiological consequences [30
]. If Green Exercise could modify ANS control during sleep, this would have important implications for health and wellbeing. Further work is also warranted to explore duration and intensity of the walks and the dose of nature itself, as well of the time of day that individuals perform such walks.