Light effects 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. Of the various factors that influence this entrainment, light exposure to the eyes is the strongest.
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:
- 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 length of light exposure influences entrainment.
- The intensity and the wavelength of light influence entrainment.
Light's effect on the circadian rhythms of all or most animals has been well documented. However, since circadian rhythms are internal functions, the influence of external factors like light and an individual's sensitivity to them can to some degree be regulated by internal mechanisms.
- In zebrafish, evidence of a negative regulation of light-dependent gene transcription has been found. In one study, overabundance of the enzyme catalase reduced the transcription of genes that were dependent on light, whereas inhibition of the enzyme resulted in increased transcription.
- Another study found that a deficit of the oligopeptide angiotensin in the brain of laboratory rats resulted in delayed adjustment to changes in the day/night pattern.
- Similarly, deficits of TrkB tyrosine kinase in mice, a receptor for brain-derived neurotrophic factor (BDNF), result in a decrease of the ability to entrain to shifts in the day–night cycle.
Internal conditions may thus sway the effectiveness of entrainment to light. All mechanisms behind the process are not yet fully understood.
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.
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