Light effects on circadian rhythm are the effects that light has on circadian rhythm.
Most animals and other organisms have "built-in clocks" in their brains that regulate the timing of biological processes and daily behavior. These "clocks" are known as circadian rhythms. They allow maintenance of these processes and behaviors relative to the 24-hour day/night cycle in nature. Although these rhythms are maintained by the individual organisms, their length does vary somewhat individually. Therefore, they must, either continually or repeatedly, be reset to synchronize with nature's cycle. In order to maintain synchronization ("entrainment") to 24 hours, external factors must play some role. The human circadian rhythm occurs typically in accordance with nature's cycle. The average activity rhythm cycle is 24.18 hours in adulthood but is shortened as age increases. One of the various factors that influence this entrainment is light exposure to the eyes. When an organism is exposed to a specific wavelength of light stimulus at certain times throughout the day, the hormone melatonin is suppressed, or prevented from being secreted by the pineal gland.
Mechanism
Light first passes into a mammal's system through the retina, then takes one of two paths: the light gets collected by rod cells and cone cells and the retinal ganglion cells (RGCs), or it is directly collected by these RGCs.
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 differently 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 shown to be more sensitive at lower wavelengths than the photopic luminous efficiency function which is based on sensitivity to cone receptor.
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 which 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.
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 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.