Light and the Circadian Cycle

sleep-99042_640A circadian cycle or rhythm is any biological process that displays an internal, entrainable oscillation of about 24 hours. Entrainment, within the study of chronobiology, occurs when rhythmic physiological or behavioral events match their period and phase to that of an environmental oscillation. Although circadian rhythms are endogenous (“built-in”, self-sustained), they are adjusted (entrained) to the local environment by external cues called zeitgebers, the most important of which is the day/night cycle.

Circadian rhythm is present in the sleeping and feeding patterns of animals, including human beings. There are also clear patterns of core body temperature, brain wave activity, hormone production, cell regeneration, and other biological activities. The activity/rest (sleep) cycle in animals, including humans, is only one set of circadian rhythms that normally are entrained by environmental cues, but is vitally important to the timing of essential, health maintaining, metabolic processes in the body.

In humans, the primary circadian “clock” is located in the suprachiasmatic nucleus (or nuclei) (SCN), a pair of distinct groups of cells located in the hypothalamus. The retina of the eye contains “classical” photoreceptors (“rods” and “cones“), which are used for conventional vision. But the retina also contains specialized ganglion cells that are directly photosensitive, and project directly to the SCN, where they help in the entrainment of this master circadian clock. The SCN takes the information on the lengths of the day and night from the retina, interprets it, and passes it on to the pineal gland, a tiny structure shaped like a pine cone and located on the epithalamus. In response, the pineal secretes the hormone melatonin.

Secretion of melatonin peaks at night and ebbs during the day and its presence provides information about night-length. The melatonin signal forms part of the system that regulates the sleep–wake cycle by chemically causing drowsiness and lowering the body temperature. Besides its function as synchronizer of the biological clock, melatonin is a powerful free-radical scavenger and wide-spectrum antioxidant. Melatonin is an antioxidant that can easily cross cell membranes and the blood–brain barrier, providing important protection against mitochondrial oxidative stress. Production of melatonin by the pineal gland is inhibited by light to the retina and permitted by darkness. Its onset each evening is called the dim-light melatonin onset (DLMO). It is principally blue light, around 460 to 480 nanometers, that suppresses melatonin, with the effect being proportional to the light intensity and length of exposure.

Until recent history, humans in temperate climates were exposed to few hours of (blue) daylight in the winter; their fires gave predominantly yellow spectrum light. The incandescent light bulb widely used in the twentieth century produced relatively little blue light. The current use of Daylight type light bulbs, fluorescent lights, compact fluorescent light bulbs, LED light bulbs, LED computer screens and LED TV screens all emit high amounts of blue spectrum light.

Chronic use of artificial blue light is equivalent to chronic excessive carbohydrates because both contain excessive photoelectric energies. This alters mitochondrial functioning after a period of time because our mitochondria evolved expecting a seasonal variation of energies in both photons and electrons. It destroys the brain’s ability to properly signal environmental signs to our cellular machinery. It affects this molecular machinery pretty quickly by fast forwarding our circadian chemical clocks to light. These clocks are all biologically tied to the cell cycle that controls growth.

Normally after four hours of darkness there is a surge of the pituitary secretion of Prolactin, from approximately 12 to 2 AM. The prolactin release is required for proper control of sleep stages and yoking sleep and metabolism, with the primary benefit being as the signal the hypothalamus uses to release growth hormone from 2 AM to 5 AM during sleep stages 2-4. This allows the process of autophagy to be maximally efficient during sleep.

Autophagy is cellular renewal, with the synthesis, degradation and recycling of cellular components and is critical to the healthy maintenance of the body. If autophagy is poor, an individual suffers more diseases and ages faster because their sleep is uncoupled from their metabolism. They are chronically using old protein enzymes that have not been renewed during sleep.  When old proteins and enzymes are used chronically, the result is altered cell signaling, which leads to disease generation.

The surge of Prolactin is normally quite strong in normal darkness but is significantly diminished in artificially lit environments after sunset. This reduction has vitally important effects on human health. Prolactin release is coordinated with sleep cycles where autophagy is at its highest efficiency and where Growth Hormone is released. Growth hormone (GH or HGH), also known as somatotropin or somatropin, is a peptide hormone that stimulates growth, cell reproduction and regeneration in humans and other animals. When we see chronic lowered prolactin surges we also see lower growth hormone secretion during the anabolic phases of sleep. Lowered chronic GH secretion directly affects cardiac and skeletal muscle function because the process of autophagy is made less efficient as we age. Lowered GH and the sex steroid hormones at sleep lead to loss of cardiac function. When growth hormone is not released in normal amounts, it also decreases our lean muscle mass, and increases our body fat percentage in all our organs and in our body. This leads to slowly declining organ dysfunction and poor body composition. Prolactin is the trigger for GH release at night. GH decreases abdominal fat cells and simultaneously increases your lean muscle mass and allows for major protein synthesis. Its levels fall off a cliff for most women after age 40 and for men after age fifty and there is a corresponding drop in the efficiency of sleep and autophagy. This is why older people sleep less than younger people. If this Prolactin release is diminished, cell regeneration is severely impacted while lower DHEA levels result along with higher inflammation due to an increase in inflammatory cytokines. These cytokines are destructive to cell membrane and nuclear signaling, potentially leading to various disease states as well as aging diseases.  DHEA has been shown to prevent chronic inflammation and it slows the aberrant signaling that is commonly found in the immune system when it is turned on by any pathogen, self or foreign.

An added complication is that the normal large circadian prolactin surge we should see at around midnight, after leptin enters the brain, does not happen if the patient has leptin resistance, sleep apnea, or has eaten food too close (within 3-4 hours) to bedtime. This blocks leptin’s ability to enter the brain because insulin spikes block leptin from binding to its receptor in the hypothalamus.

This makes it clear that sleep, which is correctly yoked to the circadian day/night, light/dark cycle is tied directly to optimal cell cycle functioning and directly to cell mediated immunity. It is also evident that sleep directly effects the chronic diseases of aging and likely plays a role in cancer development.

In order to achieve and maintain optimal health, one should align their lifestyle and sleeping habits as closely as possible to the light/dark circadian cycle by utilizing the following suggestions:

1) Use a minimal amount of artificial lighting. 

2) If artificial lighting is found to be necessary, one should utilize yellow spectrum lighting sources in place of blue spectrum lighting.

3) If utilizing blue lighting sources is mandatory, wearing glasses that block blue light in the hours before bedtime may decrease melatonin loss.

4) Avoid looking at bright screens beginning two to three hours before bed.

5) Refrain from eating three to four hours before sleep in order to prevent an insulin spike that will block leptin from performing its role in the circadian metabolic process.

6) The sleeping area should ideally be free of any artificial lighting.

7) If night lights are necessary, use red lights which have the least effect on melatonin production.

8) If necessary, the ingestion of small amounts of melatonin several hours before sleep, can also be used to shift the circadian clock earlier, thus promoting earlier sleep onset.

A number of studies have concluded that a short period of sleep during the day, such as a power-nap, does not have any measurable effect on normal circadian rhythms but can decrease stress and improve productivity.

It should be evident from the information above that anyone who is interested in achieving and maintaining optimal health, should avoid working repeated night-shift hours, whether on alternating nights or not. This sort of circadian mismatch will most certainly lead to serious health issues the longer it continues, and will necessitate a prolonged period of time for health recovery once it is discontinued.


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