Light effects on circadian rhythm are the response of circadian rhythms to light. Most human beings, animals and other living organisms have a biological clock that synchronizes their physiology and behaviour with the daily changes in the environment. The physiological changes that follow these clocks are known as circadian rhythms. Because the endogenous period of these rhythms is approximately 24 hours, these rhythms must be reset by external cues to synchronize with the daily cycles in the environment. This process is called entrainment. One of the most important cues to entrain circadian rhythms is light.
Mechanism Light first passes into a mammal's circadian system through the retina, then takes one of two paths: the light gets collected by rod cells and cone cells that project to a small number of the retinal ganglion cells (RGCs) that are also intrinsically light sensitive. The RGCs use the photopigment melanopsin to absorb the light energy. Specifically, this class of RGCs being discussed is referred to as "intrinsically photosensitive", which just means they are sensitive to light. There are five known types of intrinsically photosensitive retinal ganglion cells (ipRGCs): M1, M2, M3, M4, and M5. Each of these different ipRGC types have different melanopsin content and photosensitivity. These connect to amacrine cells in the inner plexiform layer of the retina. Ultimately, via this retinohypothalamic tract (RHT) the suprachiasmatic nucleus (SCN) of the hypothalamus receives light information from these ipRGCs. The ipRGCs serve a different function than rods and cones, even when isolated from the other components of the retina, ipRGCs maintain their photo-sensitivity and as a result can be sensitive to different ranges of the light spectrum. Additionally, ipRGC firing patterns may respond to light conditions as low as 1 lux whereas previous research indicated 2500 lux was required to suppress melatonin production. Circadian and other behavioral responses have been shown to be more sensitive at lower wavelengths than the photopic luminous efficiency function that is based on sensitivity to cone receptors. The core region of the SCN houses the majority of light-sensitive neurons. From here, signals are transmitted via a nerve connection with the pineal gland that regulates various hormones in the human body. There are specific genes that determine the regulation of circadian rhythm in conjunction with light. When light activates NMDA receptors in the SCN, CLOCK gene expression in that region is altered and the SCN is reset, and this is how entrainment occurs. Genes also involved with entrainment are PER1 and PER2. Some important structures directly impacted by the light–sleep relationship are the superior colliculus-pretectal area and the ventrolateral pre-optic nucleus. The progressive yellowing of the crystalline lens with age reduces the amount of short-wavelength light reaching the retina and may contribute to circadian alterations observed in older adulthood.
Effects
Primary All of the mechanisms of light-affected entrainment are not yet fully known, however numerous studies have demonstrated the effectiveness of light entrainment to the day/night cycle. Studies have shown that the timing of exposure to light influences entrainment; as seen on the phase response curve for light for a given species. In diurnal (day-active) species, exposure to light soon after wakening advances the circadian rhythm, whereas exposure before sleeping delays the rhythm. An advance means that the individual will tend to wake up earlier on the following day(s). A delay, caused by light exposure before sleeping, means that the individual will tend to wake up later on the following day(s). The hormones cortisol and melatonin, known as the "sleep hormone", are affected by the signals light sends through the body's nervous system. These hormones help regulate blood sugar to give the body the appropriate amount of energy that is required throughout the day. Cortisol levels are high upon waking and gradually decrease over the course of the day, melatonin levels are high when the body is entering and exiting a sleeping status and are very low over the course of waking hours. The earth's natural light-dark cycle is the basis for the release of these hormones. The length of light exposure influences entrainment. Longer exposures have a greater effect than shorter exposures. Consistent light exposure has a greater effect than intermittent exposure. In rats, constant light eventually disrupts the cycle to the point that memory and stress coping may be impaired. The intensity and the wavelength of light influence entrainment. Dim light can affect entrainment relative to darkness. Brighter light is more effective than dim light. In humans, a lower intensity short wavelength (blue/violet) light appears to be equally effective as a higher intensity of white light. Exposure to monochromatic light at the wavelengths of 460 nm and 550 nm on two control groups yielded results showing decreased sleepiness at 460 nm tested over two groups and a control group. Additionally, in the same study but testing thermoregulation and heart rate researchers found significantly increased heart rate in 460 nm light over the course of a 1.5-hour exposure period. In a study done on the effect of lighting intensity on delta waves, a measure of sleepiness, high levels of lighting (1700 lux) showed lower levels of delta waves measured through an EEG than low levels of lighting (450 lux). This shows that lighting intensity is directly correlated with alertness in an office environment. Humans are sensitive to light with a short wavelength. Specifically, melanopsin is sensitive to blue light with a wavelength of approximately 480 nm. The effect this wavelength of light has on melanopsin leads to physiological responses such as the suppression of melatonin production, increased alertness, and alterations to the circadian rhythm.
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