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The Circadian Rhythm and How it Works

Clocks in Bran

The Overall Picture

An individual’s environmental cues, also known as zeitgebers, drive the endogenous process of the circadian rhythm. These cues can include the timing of sleep, meals, work/social interactions.

Disturbance of this can have significant detrimental effects on overall health. Non-rhythmic regulations of core body temperature, cortisol levels, and melatonin secretion are all indicators of these irregularities. Cortisol secretion is rhythmic (due to hypothalamus), overproduction of cortisol can inhibit one’s ability to induce sleep. Likewise, the underproduction of melatonin can negatively impact one’s ability to fall asleep.

How Day to Day we see a Change in the Circadian Rhythm.

Variable: LIGHT

  • About 75% of the world’s population is exposed to artificial light at night
  • It has been estimated that individuals in modern societies commonly experience light intensity levels over twice as high between sunset and sleep compared to exposure to only natural light.
  • Some of these devices emit monochromatic blue light (λmax, 460–480 nm), to which intrinsically photosensitive retinal ganglion cells are especially sensitive
  • Irradiance levels as low as 2 μW/cm2 of such light suppress nocturnal melatonin production

As a result, nighttime exposure to even low levels of light from e-Book devices delays sleep and dim-light melatonin onset, reduces melatonin synthesis, and impairs next-morning alertness.

Variable: Meal Time

  • Deals with the peripheral circadian rhythm
  • Less on sleepiness, latency, etc. but on other bodily functions.
  • Consequently, synchrony within and between tissues demonstrate daily rhythms. The cell autonomous oscillator itself and a significant portion of the genome in each tissue also indirectly respond to food intake and light/dark cycles in a time of the day specific manner. As a result, the circadian system is a master integrator of both the internal state of the organism and the organism’s interaction with nutrition and ambient light.
  • While the circadian system’s plasticity towards change in ambient lighting or food availability has been an advantage in nature to adapt to different seasons, such plasticity can become a liability in modern society where both light and food are available around the clock.


The invention of electrical lighting, almost all humans voluntarily override this natural mechanism of diurnal rhythm by self-selecting a sleep-wake pattern that suits their schedule, which leads to associated alterations in feeding and fasting. Such chronic disruption of diurnal rhythms can compromise health through multiple discrete mechanisms. Reduced sleep can disrupt metabolic homeostasis by mechanisms that are yet to be fully understood:

  • Light at night suppresses sleep and promotes extended wakefulness, thus allowing ingestive behavior to continue late into the night
  • This extended period of eating may contribute to increased food intake that often correlates to the human lifestyle.
  • Eating at a sub-optimal time of the 24hr day can promote excessive energy storage instead of expenditure.
  • Nutrition quality can also impact hunger, satiety, and hedonic drive for food intake and thereby affect the daily eating patterns, which in turn can impact the robustness of circadian oscillators in various organs.
  • Conversely, maintaining a defined daily feeding-fasting rhythm, as in time-restricted feeding (TRF), can prevent or attenuate several chronic diseases.
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