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 reason why entrainment occurs in humans is because each individual's circadian rhythm is longer than 24 hours (majority of population) or shorter than 24 hours (minority of population). Of the various factors that influence this entrainment, light exposure to the eyes is the strongest. Melatonin plays a large role in effects of light on circadian rhythms. When an organism is exposed to a light stimulus, the hormone melatonin is suppressed, or prevented from being secreted by the pineal gland.
Contents
Demonstrated effects
All of the mechanisms of light-effected 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:
Internal pathways and mechanisms
Light first passes into a mammal's system through the retina, then takes one of two paths: the light either gets collected by rod cells and cone cells before being transmitted to 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. 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 ipRCGs.
The core region of the SCN houses the majority of light-sensitive neurons. From here, signals must reach the rest of the SCN in order for circadian shifts, or entrainment, to occur.
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.
Secondary effects
While light has direct effects on circadian rhythm, there are indirect effects seen across studies. Seasonal affective disorder creates a model in which decreased day length during autumn and winter increases depressive symptoms. A shift in the circadian phase response curve creates a connection between the amount of light in a day (day length) and depressive symptoms in this disorder. Light seems to have therapeutic antidepressant effects when an organism is exposed to it at appropriate times during the circadian rhythm, regulating the sleep-wake cycle.
In addition to mood, learning and memory become impaired when the circadian system shifts due to light stimuli, which can be seen in studies modeling jet lag and shift work situations. Frontal and parietal lobe areas involved in working memory have been implicated in melanopsin responses to light information.
In response to light exposure, alertness levels can increase as a result of suppression of melatonin secretion. A linear relationship has been found between alerting effects of light and activation in the posterior hypothalamus.
Disruption of circadian rhythm as a result of light also produces changes in metabolism.
Other factors
Although many researchers consider light to be the strongest cue for entrainment, it is not the only factor acting on circadian rhythms. Other factors may enhance or decrease the effectiveness of entrainment. For instance, exercise and other physical activity, when coupled with light exposure, results in a somewhat stronger entrainment response. Other factors such as music and properly timed administration of the neurohormone melatonin have shown similar effects. Numerous other factors affect entrainment as well. These include feeding schedules, temperature, pharmacology, locomotor stimuli, social interaction, sexual stimuli and stress.
Treatment
Dark therapy before bedtime, where light that triggers melanopsin receptors is filtered out with eyewear, is considered an effective method to promote earlier production of melatonin and improve sleep.