The circadian rhythm is a "biological clock" that governs physical, mental and behavioral changes -- including the rate of your heart -- on a roughly 24-hour clock. It is controlled by a group of interacting molecules in the cells throughout the body, which are coordinated by an area in the brain known as the suprachiasmatic nucleus, or SCN. The SCN contains about 20,000 nerve cells located in the hypothalamus region of the human brain.
Circadian Rhythm
The circadian rhythm affects the functions of the body by changing, among other things, sleep-wake cycles, hormone release and body temperature. The SCN is located just above the optic nerve; when there is less light, the SCN produces melatonin, which makes you drowsy. Genes have been proven to direct the circadian rhythm, so there may be a slight variability between different individuals. Together these factors produce a precisely orchestrated synchronization, but they are entrained --- meaning reset daily --- to the 24-hour day/night cycle by the external time cues, which primarily include the variations in sunlight and the increase in environmental and human activity around us.
24-Hour Changes
Your resting heart rate tends to fluctuate according to a regular rhythm that governs both mental and physical activity levels. Nocturnal nervous system activity diminishes during sleep, and it is at this time when the heart rate tends to reach its lowest point. Upon awakening, then, the heart rate increases and peaks in the morning period. In most hypertensive patients, there is a substantial rise in blood pressure upon awakening that is called the morning or AM surge.
Cellular Activity
The heartbeat corresponds to the rate at which molecules are circulated throughout the body. Your cells need more oxygen and nutrients for energy during periods of heightened cellular activity. The circadian rhythm is merely a way for the nerve cells in the brain to regulate the activities of the cell, and therefore your heart rate, based upon the regular 24-hour cycle.
Exercise
A 2009 study published in the journal "Chronobiology International" found that aerobic exercise resulted in a significant decrease in heart rate throughout the 24-hour cycle, particularly during the daytime period. This effect was most pronounced in the morning. However, there were no significant changes at night. A lower heart rate generally corresponds to a more efficient heart. It takes fewer beats to perform the same amount of circulation, so it is therefore a factor of good health.
References
- National Institute of General Medical Sciences: Circadian Rhythms Fact Sheet
- "Hypertension"; Plasma Epinephrine and Norepinephrine Concentrations of Healthy Humans Associated With Nighttime Sleep and Morning Arousal; Christoph Dodt, Ulrike Breckling, Inge Derad, Horst Lorenz Fehm, Jan Born; 1997
- "Chronobiology International": Effects of Aerobic Exercise on the Circadian Rhythm of Heart Rate and Blood Pressure; Hideyuki Shiotani, Yoichiro Umegaki, Maiko Tanaka, Madoka Kimura, Hiroshi Ando; December 2009
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