Measures which are robust indicator of overall biological functioning may be intended as summary parameters emergent from the physiological network complexity. Meaningful and valid whole-system indicators, obtained by feasible methods, can be of crucial importance and interest. As a matter of fact, heart rate variability (HRV) has been largely used as a sign of overall system adaptability and health (1). However, the specific response of HRV as an emergent property of interdependent regulatory systems operating at different time scales to adapt to environmental and psychological stressors and stimuli (2) has been poorly addressed. We used time-domain, frequency-domain and non-linear metrics of HRV in two diverse studies; the acquisition system consisted of a customized lightweight chest strap, based on a sensor with a sampling frequency of 500 Hz; ECG was acquired during 5 or 7 minutes in the supine position; both studies adhered to ethical standards and participants provided their written, informed consent. The Himalayan study was part of the “Kanchenjunga Exploration & Physiology” project: six Italians, five men and one woman (44 ± 15 years, 25.81 ± 3.25 kg/m2), lowland dwellers, and six Nepalese, all males (30 ± 8 years, 24.36 ± 4.70 Kg/m2), lowland dwellers and altitude porters, completed 300 km of a Himalayan trek. ECG analysis was conducted at baseline, and before (bHA) and after (aHA) the high-altitude circuit (up to 5,143 m). SDNN, pNN50 and CVI were lower at baseline than at bHA (47.8 ± 24.0 vs 74.6 ± 37.6 ms, p=.010; 11.6 ± 14.0 vs 33.1 ± 26.4 %, p=.003; 3.08 ± 0.40 vs 3.51 ± 0.52, p=.008). Higuchi fractal dimension (HFD) showed higher basal values in the Nepalese group (1.60 ± 0.04 vs 1.55 ± 0.03, p=.041), and a tendency for highest values at bHA. It may be argued that, before the trek started, participants had greater arousal and stress. Our results also indicated a better cardiovascular resilience of porters. The musical study was a cross-over intervention study part of the “HRV & Breath Tune” project: 25 European university students, 21-to-34 years, whose 68% females and 32% males, listened – in diverse days randomly ordered – to the Mozart Sonata in A minor, K. 331 (Mozart) or the same piece consisting only of beat (Destructured), plus Control. Tendencies were found for both SD1 (p=.092, η2p=0.13) and SD2 (p=.130, η2p=0.11) – with lower values during Mozart listening – and HFD, lower during Destructured listening (1.60 ± 0.04 vs 1.62 ± 0.07 and 1.62 ± 0.08, p=.117, η2p=0.13). We argued that Control was the most relaxing condition, and Mozart allowed to maintain a bit of arousal, supporting that the “Mozart effect” acts through mood (3). We suggest using non-linear metrics, such as HFD (4), to evaluate the effect of several stressors on a plethora of populations. All in all, HRV metrics represent an intriguing hallmark of stress, disease and ageing; however, caveats should be highlighted about the use of HRV diverse metrics for feasible application, as large misuse and overuse flaw such indicators.