Light effects on circadian rhythm
Numerous organisms maintain inherent individual daily 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 continually or repeatedly be reset to synch with the natural environmental cycle. 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, light exposure to the eyes is the strongest effecter.
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:
- The timing of exposure to light influences entrainment; as seen on the phase response curve for light for a given species.
- The length of exposure influences entrainment.
- Intensity and wavelength of light influence entrainment.
Internal regulators 
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.
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 decrease the effectiveness of entrainment. For instance, physical activity like exercise when coupled with light exposure results in a somewhat stronger entrainment response. Other factors such as music and 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.
See also 
- Kolmos, E. and S.J. Davis (2007). "Circadian rhythms: Rho-related signals in time-specific light perception." Current Biology 17(18): R808–R810.
- Baehr, E.K., L.F. Fogg, et al. (1999). "Intermittent bright light and exercise to entrain human circadian rhythms to night work." American Journal of Physiology-Regulatory Integrative and Comparative Physiology 277(6): R1598–R1604.
- Hirayama, J., S. Cho, et al. (2007). "Circadian control by the reduction/oxidation pathway: Catalase represses light-dependent clock gene expression in the zebrafish." Proceedings of the National Academy of Sciences of the United States of America 104(40): 15747–15752.
- Warman, V.L., D.J. Dijk, et al. (2003). "Phase advancing human circadian rhythms with short wavelength light." Neuroscience Letters 342(1–2): 37–40.
- Duffy, J.F., R.E. Kronauer, et al. (1996). "Phase-shifting human circadian rhythms: Influence of sleep timing, social contact and light exposure." Journal of Physiology-London 495(1): 289–297.
- Ma, W.P., J. Cao, et al. (2007). "Exposure to chronic constant light impairs spatial memory and influences long-term depression in rats." Neuroscience Research 59(2): 224–230.
- Gorman, M.R., M. Kendall, et al. (2005). "Scotopic illumination enhances entrainment of circadian rhythms to lengthening Light : Dark cycles." Journal of Biological Rhythms 20(1): 38–48.
- Campos, L.A., R. Plehm, et al. (2006). "Altered circadian rhythm reentrainment to light phase shifts in rats with low levels of brain angiotensinogen." American Journal of Physiology-Regulatory Integrative and Comparative Physiology 290(4): R1122–R1127.
- Allen, G.C., X.Y. Qu, et al. (2005). "TrkB-deficient mice show diminished phase shifts of the circadian activity rhythm in response to light." Neuroscience Letters 378(3): 150–155.
- Goel, N. (2006). "An arousing, musically enhanced bird song stimulus mediates circadian rhythm phase advances in dim light." American Journal of Physiology-Regulatory Integrative and Comparative Physiology 291(3): R822–R827.
- Revell, V.L., H.J. Burgess, et al. (2006). "Advancing human circadian rhythms with afternoon melatonin and morning intermittent bright light." Journal of Clinical Endocrinology and Metabolism 91(1): 54–59.
- Salazar-Juarez, A., L. Parra-Gamez, et al. (2007). "Non-photic entrainment. Another type of entrainment? Part one." Salud Mental 30(3): 39–47.