Numerous organisms maintain inherent individual daily rhythms to biological process

Biological process

A biological process is a process of a living organism. Biological processes are made up of any number of chemical reactions or other events that results in a transformation....

es, known as circadian rhythm

Circadian rhythm

A circadian rhythm, popularly referred to as body clock, is an endogenously driven , roughly 24-hour cycle in biochemical, physiological, or behavioural processes. Circadian rhythms have been widely observed in plants, animals, fungi and cyanobacteria...

s, 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

Entrainment (chronobiology)

Entrainment, within the study of chronobiology, occurs when rhythmic physiological or behavioral events match their period and phase to that of an environmental oscillation. A common example is the entrainment of circadian rhythms to the daily light–dark cycle, which ultimately is determined by...

, 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
  Phase response curve
  A phase response curve illustrates the transient change in the cycle period of an oscillation induced by a perturbation as a function of the phase at which it is received...
  
   for light for a given species.
  • In diurnal species, exposure to bright light after wakening advances the circadian rhythm, whereas exposure before sleeping delays the rhythm.
• The length of exposure influences entrainment.
  • Longer exposures have a greater effect than shorter exposures.
  • Consistent exposure has a greater effect than intermittent exposure.
  • In rats, constant exposure eventually disrupts the cycle to the point that functions like memory and stress coping may be impaired.
• Intensity and wavelength of light influence entrainment.
  • Brighter light is more effective than dim light.
  • Dim light can effect entrainment relative to darkness.
  • In humans, low intensity short wavelength (blue/violet) light may be equally effective as high intensity white light.

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
  Catalase
  Catalase is a common enzyme found in nearly all living organisms that are exposed to oxygen, where it catalyzes the decomposition of hydrogen peroxide to water and oxygen...
  
   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
  Angiotensin
  Angiotensin, a peptide hormone, causes blood vessels to constrict, and drives blood pressure up. It is part of the renin-angiotensin system, which is a major target for drugs that lower blood pressure. Angiotensin also stimulates the release of aldosterone, another hormone, from the adrenal cortex...
  
   in the brain of laboratory rats resulted in delayed adjustment to changes in the day/night pattern.
• Similarly, deficits of TrkB tyrosine kinase
  Tyrosine kinase
  A tyrosine kinase is an enzyme that can transfer a phosphate group from ATP to a protein in a cell. It functions as an "on" or "off" switch in many cellular functions....
  
   in mice, a receptor for brain-derived neurotrophic factor
  Brain-derived neurotrophic factor
  Brain-derived neurotrophic factor, also known as BDNF, is a protein that, in humans, is encoded by the BDNF gene. BDNF is a member of the "neurotrophin" family of growth factors, which are related to the canonical "Nerve Growth Factor", NGF...
  
   (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

Melatonin

Melatonin , also known chemically as N-acetyl-5-methoxytryptamine, is a naturally occurring compound found in animals, plants, and microbes...

 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.