Timing is everything

Circadian light-dark cycles control internal body clocks which trigger and synchronize physiological processes and behaviors. The three primary rhythms controlled by the master internal clock, called the suprachiasmic nucleus, are the hormones melatonin and cortisol, and core body temperature. The master clock and these three cyclical rhythms translate environmental light into signals that control the timing of processes and behaviors as diverse as gene transcription, protein synthesis, cell division, hormone secretion, sleep, alertness, metabolic rate, hunger, physical stamina, cognitive performance, neuronal remodeling and more. Our bodies are composed of many internal clocks that keep time based on the fundament rhythms of light and dark, hot and cold, activity and rest. 

Nearly every process in the body has a specific temporal profile that is key to the integrity of the physiological or behavioral output of the process. These temporal profiles are characterized by oscillations between peak activity periods and quiet periods, and by completion of a full oscillation cycle approximately every 24 hours or some factor of 24.

Three key attributes of these oscillations determine the integrity of any given physiologic process. The first attribute is the amplitude of the oscillation which is the size of the oscillation’s swing between maximum activity level to minimum activity level.  As is discussed below, states of health and optimal physiologic function are associated with large amplitude oscillations and very distinctive, clean transitions from activity to inactivity. Conversely, compromised resilience and disease states are associated with flattened, small amplitude oscillations with indistinct, or complete loss of, maximums and minimums.

The second attribute is the period, which is the time it takes to complete a full oscillation from maximum activity to minimum activity and back to the maximum. Just as the light-dark oscillation of a day is 24 hours, so too, the period of most healthy physiological processes is approximately 24 hours. Both genetics and environmental cues influence oscillation periods, and these influences form the dynamics of each person’s unique internal clockworks.

The third key attribute is the synchrony of the oscillation with other co-dependent processes and with the external objective clock of our environment. When a physiologic process or behavior is triggered at an inappropriate time, such as being awake at 2 am, the oscillations of sleep, neuronal remodeling, protein synthesis and objective clock time become desynchronized with consequent health and performance effects.

The association between oscillatory amplitude and disease has been demonstrated by numerous studies from diverse fields. For example, several cardiology studies have shown heart failure severity and risk for myocardial infarction are significantly higher when the amplitude of 24 hour heart rate variability is low. Likewise, decreased sympathetic system amplitude due to chronic elevation of nighttime sympathetic tone (when the sympathetic valley is supposed to occur), is associated with hypertension, insomnia, anxiety and post-traumatic stress disorder. Conversely, several studies have shown the risk for heart disease is significantly lower when the amplitude of 24 hour heart rate variability is high.

Other examples of high amplitude and health benefits come from neurobiology studies that have found large amplitude core body temperature oscillations correlate with greater neuroplasticity and functional recovery following brain injury (due to higher levels of Brain Derived Neurotropic Factor - BDNF). In addition, large amplitude CBT oscillations correlate with longer disease-free live spans.  As a whole, these and other similar studies illustrate the pattern of low oscillatory amplitude correlating with disease and high amplitude oscillatory amplitude correlating with health and resilience.