Reduced heart rate variability during sleep in long-duration spaceflight.

Area of Science:

• Cardiovascular physiology
• Space medicine
• Human physiology

Background:

• Limited data exist on heart rate (HR) regulation during sleep in spaceflight.
• Sleep offers a stable supine baseline for comparing heart rate variability (HRV) using various analytical methods.

Purpose of the Study:

• To investigate the impact of long-duration spaceflight on HR and HRV during sleep.
• To compare preflight, inflight, and postflight physiological data.

Main Methods:

• Electrocardiographic waveforms from Holter monitors and accelerometer data were collected from seven astronauts.
• Data were analyzed using linear, complexity, and fractal indicators of HRV.
• Measurements were taken before, during, and after spaceflight missions up to 6 months.

Main Results:

• Heart rate (HR) remained unchanged inflight but was elevated postflight. HRV indicators significantly decreased inflight across all frequencies, with partial recovery postflight.
• Complexity and fractal properties of HR did not differ from preflight.
• Slow HR fluctuations correlated with sleep movements, partly explaining HRV reduction.

Conclusions:

• Spaceflight significantly alters periodic elements influencing HR regulation via reflex mechanisms during sleep.
• Linear methods revealed substantial HRV reduction, while complexity and fractal analyses showed no significant changes.
• Underlying system complexity and fractal dynamics of HR regulation appear preserved during spaceflight.