Bright light therapy is a treatment approach that has shown promise for managing disruptions to circadian cycles, including disruptions associated with crossing multiple time zones or rotating work shifts.

Light Therapy for Circadian Rhythm Disruptions

The process by which circadian rhythms can be reset by light. 

* Phase delay: time of sleep and wake move to later in the day, due to light exposure in the first half of the night.
* Phase advance: time of sleep and wake move to earlier in the day, due to light exposure in the second half of the night..

University of Michigan - Ann Arbor

An internal biological clock that allows animals to proceed with their circadian rhythms. Usually, the internal clock is dependent on light. However, animals will still continue their circadian rhythm even in the absence of external cues. For example, even if it is constantly light or dark, animals will proceed with their circadian rhythm. 
- free running

Endogenous clock and circadian rhythm

Luo, L. (2020). Principles of Neurobiology (2nd ed.). CRC Press. 

Principles of Neurobiology, 2nd edition (2020)

When animals continue to maintain their own cycle even when they aren't provided external cues about time of day, it is called free-running. The cycle during free-running is not exactly 24 hours. 
- changes in circadian rhythm
- hypothalamus 

Free running

Entrainment

The suprachiasmatic nucleus (SCN) of the hypothalamus regulates circadian rhythms. Light activates intrinsically photosensitive retinal ganglion cells, causing the release of glutamate onto SCN neurons. This eventually leads to the transcription of the circadian regulating genes Per1 and Per2. The SCN is referred to as the "circadian pacemaker" because it oscillates in a circadian fashion.

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