Photoentrainment (chronobiology)

In chronobiology, photoentrainment refers to the process by which an organism's biological clock, or circadian rhythm, synchronizes to daily cycles of light and dark in the environment.

[1] Photoentrainment plays a major role in maintaining proper timing of physiological processes and coordinating behavior within the natural environment.

Two prominent 20th century chronobiologists, Jürgen Aschoff and Colin Pittendrigh, both worked throughout the 1960s to model of the process of photoentrainment, and despite examining the same subject, they arrived at different conclusions.

[7]Pittendrigh developed his model based on the phase-response curve, which visualizes the effect of short light pulses on organisms that were free-running (not entrained to a zeitgeber).

Exposure to higher-intensity light was found to either extend or shorten an organism’s period depending on species, dubbed Aschoff’s rule.

[8] The molecular mechanism for photoentrainment in multicellular organisms such as in fungi and animals has been linked to the transcription-translation feedback loop (TTFL), where translated protein products influence gene transcription.

Many ecological relationships such as predator-prey interactions, pollinator behaviors, migration timing all require the synchronization of an organism’s biological clock with the 24-hour rhythm of planet.

[10] In addition to melanopsin, studies have determined from using melanopsin-knockout mice that rods and cones can also play a role in the photic responses of the SCN.

[13] Photoautotrophic cyanobacteria depend on sunlight for energy, so a failure to anticipate nighttime would threaten their ability to survive and reproduce.

Appropriate synchronization to light also facilitates the temporal separation between oxygen-sensitive nitrogen fixation and oxygen-generating photosynthesis, lest the latter would inhibit the former.

Jet lag occurs when one’s circadian rhythm is out of sync with the environment, and this is usually caused by travel across time zones.

[21] Disruption of an individual's dopamine activity due to the lack of light in the winter months is thought to be a cause of seasonal affective disorder (SAD).

DSPT occurs when one sleeps late and is unable to wake up early, resulting in a lack of entrainment to a typical working schedule.

Light therapy in the evening (that is administered before one’s body temperature reaches its low point) may help in inducing phase delay in these patients.

One study found that morning light therapy helped dementia patients with their sleep, yet functioning did not improve.

[25] Further, a different meta-analysis found that light therapy helped patients with their symptoms and did not cause any negative effects.

Phase-response curve and phase shifts under different testing conditions. Condition A shows that a light pulse in the subjective day does not result in a phase shift. Condition B and C shows that a light pulse in the early subjective night results in a phase delay. Condition D shows that a light pulse in the late subjective night results in a phase advance. [ 8 ]