Light effects on circadian rhythm

Numerous organisms maintain inherent individual rhythms to biological processes, known as circadian rhythms, that assist the organism in maintaining functional periodicity relative to the 24 hour day/night cycle of the earth. These rhythms are maintained by the individual organisms, but due to variable individuality and environmental pressures, must be continually reset to synch with the natural environmental cycle. ^[1] In order for this to be accomplished, external factors must play some role in the synchronization, or entrainment, of the internal circadian rhythm with the external environment. Of the various factors that influence this entrainment, exposure to light is the strongest effecter. ^[2][3][4]

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Demonstrated Effects

All of the mechanisms of light effected entrainment are not fully known yet, however numerous studies have demonstrated the effectiveness of light entrainment to the day/night cycle. Studies have shown that:

• The time of exposure to light influences entrainment
  • Exposure to bright light after wakening advances the circadian rhythm, whereas exposure before sleeping delays the rhythm. ^[4][5]
• The length of exposure influences entrainment
  • Longer exposures have a greater effect than shorter exposures ^[5]
  • Consistent exposure has a greater effect than intermittent exposure ^[2]
  • Constant exposure eventually disrupts the cycle to the point that other functions like memory and stress coping may be impaired. ^[6]
• Intensity and wavelength of light influence entrainment
  • Brighter light is more effective than dim light ^[5]
  • Dim light can effect entrainment relative to darkness ^[7]
  • Low intensity ultraviolet light was equally effective as high intensity white light ^[4]

Internal Regulators

Light’s effect on the circadian rhythm has been well documented. However, it is worth noting that since circadian rhythms are internal functions, the influence of external factors like light can be regulated in some degree by internal mechanisms. For example, researchers found evidence of a negative regulation of light-dependent gene transcription in zebrafish. In this 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. ^[3] Another study found that a deficit of the oligopeptide angiotensin in the brain resulted in delayed adjustment to changes in the day/night pattern of laboratory rats. ^[8] Similarly, deficits of TrkB tyrosine kinase, a receptor for brain-derived neurotrophic factor (BDNF), also result in decrease of entrainment to shifts in the day/night cycle. ^[9] These studies are only a few examples to demonstrate the importance of internal regulators to the effectiveness of entrainment to light. All mechanisms behind the process are not yet fully understood.

Other Factors

Although many researchers consider light to be the strongest cue for entrainment, it is by no means the only factor acting on circadian rhythms. Other factors may enhance or detract the effectiveness of entrainment. For instance, physical activity like exercise when coupled with light exposure results in a somewhat stronger entrainment response. ^[2] Other factors such as music and dosage with melatonin have shown similar effects. ^[10][11] Numerous other factors affect entrainment as well. Temperature, pharmacology, locomotor stimuli, social interaction, sexual stimuli, stress, and many others have also been shown to effect circadian entrainment, even in absence of cues from light. ^[12]

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