Editing Jet lag (section)

=== Entrainment to a local time ===
[[Entrainment (chronobiology)|Entrainment]] to a local time zone refers to the synchronization of an individual’s internal circadian rhythms to the external environment's 24-hour cycle, particularly in relation to light-dark cues ([[Zeitgeber|zeitgebers]]). This process is critical for overcoming jet lag, which occurs when a person's endogenous circadian timing becomes misaligned with the new local time following transmeridian travel.

Circadian rhythms in humans are generated by the [[Suprachiasmatic nucleus|suprachiasmatic nuclei]] (SCN) of the hypothalamus and are typically slightly longer than 24 hours, requiring daily resetting by external cues to maintain alignment with the environment.<ref name="doi.org">{{cite journal |last1=Arendt |first1=Josephine |title=Managing Jet Lag: Some of the Problems and Possible New Solutions. |journal=Sleep Medicine Reviews |date=2009 |volume= 13| issue =  4 |pages=249–56 |doi=10.1016/j.smrv.2008.07.011 |pmid=19147377 |url=https://doi.org/10.1016/j.smrv.2008.07.011.}}</ref> Light is the most powerful zeitgeber, and its timing, intensity, and spectral composition are key determinants of circadian phase adjustments.<ref name="doi.org"/>
The effect of light on circadian rhythms is described by the [[Phase response curve|Phase Response Curve]] (PRC), which illustrates how light exposure at different subjective times produces phase advances or delays. A phase-advance happens when your internal clock shifts to an earlier time—you go to sleep and wake up earlier than usual. A phase-delay happens when your internal clock shifts to a later time—you go to sleep and wake up later than usual. Light exposure in the early biological morning tends to advance the circadian phase, whereas exposure in the late biological evening delays it.<ref name="Adapting to Phase Shifts, I. An Exp">{{cite journal |last1=Deacon |first1=S |title=Adapting to Phase Shifts, I. An Experimental Model for Jet Lag and Shift Work. |journal=Physiology & Behavior |date=May 1996 |volume= 59| issue =  4–5 |pages=665–73 |doi=10.1016/0031-9384(95)02147-7 |pmid=8778850 |url=https://doi.org/10.1016/0031-9384(95)02147-7.}}</ref>

When individuals arrive in a new time zone, their internal biological night may still coincide with local daytime hours. Adaptation thus requires a shift in circadian phase to realign internal rhythms with the external light-dark cycle. For example, a study using bright light exposure (1,200–3,000 lux) demonstrated that strategically timed light can induce phase shifts of several hours, facilitating adaptation to new time zones.<ref name="Adapting to Phase Shifts, I. An Exp"/>

Entrainment is also influenced by additional zeitgebers such as meal timing, scheduled activity, and [[Chronopharmacology|chronopharmacological agents]] like melatonin. [[Melatonin]], a hormone secreted during biological night, exhibits a PRC that is roughly opposite to that of light: it advances circadian rhythms when taken in the afternoon or early evening and delays them when taken in the morning. This complementarity allows for the combined use of light and melatonin to optimize re-entrainment.

However, individual variability plays a substantial role. Factors such as age, diurnal preference (chronotype), and genetic polymorphisms (e.g., in the [[Period (gene)|PER gene]]) can affect how quickly one adapts, also in relation to the direction of travel (east-west or west-east).<ref name="Circadian Phase Response Curves to">{{cite journal |last1=Kripke |first1=Daniel F |title=Circadian Phase Response Curves to Light in Older and Young Women and Men. |journal=Journal of Circadian Rhythms |date=July 2007 |volume= 5|page=4 |doi=10.1186/1740-3391-5-4 |doi-access=free |pmid=17623102 |pmc=1988787 }}</ref> Older adults tend to have an earlier phase angle of entrainment and may show a leftward shift in their PRC, meaning their window for phase shifting occurs earlier relative to clock time than in younger adults.<ref name="Circadian Phase Response Curves to"/>