Sleep and the importance of sleep has been a hot topic for some time. New studies are routinely published on the effects of insufficient sleep on human health. Many of these studies focus on the damaging effects of even small amounts of artificial light exposure to our sleep quality, particularly blue light. Limiting our exposure to artificial light after dark is a key component of sleep hygiene. However, evidence suggests that the consequences of disrupting our circadian rhythms with light exposure are more systemic than we originally understood, resulting in detrimental effects on both physiology and behavior. Upsetting the homeostatic photo-cycles can cause sleep disturbances, irritability, depression, lack of attention, weight gain, gastrointestinal problems and even the tendency to develop cancer.
Here is a very basic explanation of how our body systems interact with light:
The suprachiasmatic nucleus (SCN), located in the hypothalamus, functions as our “master clock”. It works in conjunction with the endocrine system to regulate the hormones that signal our bodies to perform internal processes. The SCN “coordinates daily sleep-wake cycles, metabolic processes, hormonal release, and in general the temporal order of all the body physiology.” The SCN is stimulated by light. Light enters the eye and activates neurons in the retina that convert the light particles to electrical signals, which travel to the SCN. When it senses light, it signals the body to produce cortisol (a stress hormone) that warms the body and wakes it up. When darkness falls, the SCN signals the body to produce the sleep hormone melatonin, which cools the body down, prepares it for rest and stimulates DNA repair.
During sleep, a series of physiological events occur which help the body recover and prepare for the next day. Cellular regeneration and repair are activated by human growth hormones. Leptin, insulin and ghrelin, our hunger hormones, are regulated during sleep, which is why we may find ourselves more hungry than normal after a poor night’s sleep. Serotonin, a neurotransmitter believed to regulate mood and energy, decreases in levels during sleep. Studies show that the circadian cycle controls from ten to fifteen percent of our genes, so it is clearly an important part of our bodies’ functioning.
However, the modern problem is that our hours of darkness are becoming fewer and fewer, thanks to artificial light. The light intensity used for illuminating our interior spaces, particularly that of blue light, is sufficient to alter the biological clock and circadian rhythms. Blue light is especially damaging because it causes oxidative stress without the counter-balancing effects of the near-infrared frequencies found in natural light. Even just a few minutes of artificial light, including the light from mobile devices and computer screens, will suppress melatonin enough to inhibit sleep processes. Recent studies show that a night-time light exposure will shut off melatonin production for four hours. In other words, when our retinas are continually exposed to light after dark, our “off switch” is never really activated. If indoor conditions more closely approximate the natural light rhythms, the melatonin takes over slowly and the body gradually begins the restorative process of sleep. The same holds true for waking in the night during sleep: if you turn on the light during a trip to the bathroom, the light will activate your body’s waking processes. (It is interesting to note that even too much natural light at night can be detrimental. People who live in areas of the world like Siberia and Greenland where they experience prolonged periods of sunlight at night have high rates of suicide, depression and alcoholism).
Our circadian rhythms are synchronized to periods of light and dark, and they control systems that tend to cellular regeneration and recovery. Getting enough sunlight is critical to our body’s energy production systems, but periods without light are equally critical for regeneration and recovery. It follows that the more closely we synchronize our lives with natural sunlight cycles, the better our bodies can function.
”Endocrine Effects of Circadian Disruption”, Tracy A. Bedrosian, Laura K. Fonken, and Randy J. Nelson, Annual Review of Physiology, vol 78, February 2016.
“Disruption of Circadian Rhythms: A Crucial Factor in the Etiology of Depression”, Roberto Salgado-Delgado, Araceli Tapia Osorio, Nadia Saderi, and Carolina Escobar, Depression Research and Treatment, August 2011.
“Circadian Disruption Leads to Loss of Homeostasis and Disease”, Carolina Escobar, Roberto Salgado-Delgado, Eduardo Gonzalez-Guerra, Araceli Tapia Osorio, Manuel Angeles-Castellanos, and Ruud M Buijs, Sleep Disorders, 2012.
“How Blue LEDs Affect Sleep”, Alina Bradford, Live Science, February 26, 2016