Correlations between the External Respiration Indicators and the Heart Rate Variability in Students of Different Chronotypes during Their Mental Activities

We analyzed the changes and correlation relationships in external respiration parameters and heart rate variability (HRV) due to a mental load of a monotonous nature (a battery of tests for attention and mental calculation), in students with the morning (“larks”), afternoon (“pigeons”), and evening (“owls”) chronotypes. It has been shown that, under the mental load in the morning, afternoon, and evening hours, the respiratory minute volume and the inspiration flow volume rate decrease, as well as the HRV parameters, such as heart rate (HR), the index of sympathetic activity, and the stress-index decrease, though to different degrees in different chronotypes. The total spectral power of cardiac rhythm fluctuations and its separate frequency components uncertainly change in the morning, afternoon, and evening chronotypes. In general, the number of cardiorespiratory effects and their expressiveness under the load is larger in “pigeons” than in “larks” and “owls.” The correlation relationships between respiration and HRV under the conditions of a monotonous load prevail in “larks,” in particular, their closer relationships are observed during evening hours, whereas the correlations in “pigeons” and “owls” dominate in their morning work. The obtained data allow us to suggest that by the optimality of adaptation responses on the part of external respiration and HRV in the conditions of mental monotonous activities in hours not coinciding with their chronotype, the first place belongs to “pigeon” students, “owls” are the second, and the third place is left to “larks.”