Light and diurnal cycle affect autonomic cardiac balance in human; possible role for the biological clock

Area of Science:

• Cardiovascular Physiology
• Chronobiology
• Autonomic Nervous System

Background:

• The morning hours show increased cardiovascular events, potentially linked to sympatho-vagal imbalance.
• Understanding the biological clock's role in autonomic cardiac control is crucial for explaining these risks.

Purpose of the Study:

• To investigate endogenous diurnal rhythms in cardiac autonomic control.
• To examine circadian phase-dependent effects of moderate light on heart rate regulation.

Main Methods:

• Healthy volunteers underwent continuous monitoring of heart rate (HR) and cardiac vagal and sympathetic tone.
• Measurements were taken across the day-night cycle under controlled resting conditions and varying light intensities.
• Validation confirmed the setup's accuracy in estimating endogenous circadian rhythms.

Main Results:

• A distinct diurnal rhythm was observed in resting heart rate, primarily driven by circadian variations in vagal cardiac tone.
• Early morning light exposure led to an increase in heart rate.
• This light-induced heart rate increase involved a contribution from heightened sympathetic cardiac tone.

Conclusions:

• Circadian rhythms in vagal tone are the primary drivers of diurnal heart rate variations.
• Sympathetic nervous system activity is modulated by light, particularly in the morning, influencing heart rate.
• These findings enhance understanding of the interplay between the biological clock, light, and cardiac autonomic control.