Editing Jet lag (section)

==Cause==
Jet lag is a [[chronobiological]] problem,<ref>{{Cite journal |last1 = Waterhouse |first1 = J. |title = Jet-lag and shift work: (1). Circadian rhythms |journal = Journal of the Royal Society of Medicine |volume = 92 |issue = 8 |pages = 398–401 |year = 1999 |pmid = 10656004 |pmc = 1297314 |doi = 10.1177/014107689909200804 }}</ref> similar to issues often induced by [[shift work]] and [[circadian rhythm sleep disorder]]s.  During jet lag, there is a shift in the sleep-wake cycle, disrupting the coordinated regulation of the [[suprachiasmatic nucleus|suprachiasmatic nucleus (SCN)]] of the hypothalamus. The output of the SCN influences oscillatory sleep and arousal controls, which can later lead to an effect on daily sleep-wake behavior.<ref>{{cite journal |last1=M Vosko|first1=Andrew |last2=S Colwell | first2=Christopher | last3=Y Avidan | first3=Alon|title=Jet lag syndrome: circadian organization, pathophysiology, and management strategies. |journal=Nature and Science of Sleep |date=19 August 2010 |volume=2 |pages=187–198 |doi=10.2147/NSS.S6683 |doi-access=free |pmid=23616709 |pmc=3630947 }}</ref> When travelling across a number of time zones, a person's [[body clock]] (circadian rhythm) will be out of synchronisation with the destination time, as it experiences daylight and darkness contrary to the rhythms to which it was accustomed. The body's natural pattern is disturbed, as the rhythms that dictate times for eating, sleeping, [[Hormone#Regulation|hormone regulation]], [[Thermoregulation|body temperature variation]], and other functions no longer correspond to the environment, nor to each other in some cases. To the degree that the body cannot immediately realign these rhythms, it is jet lagged.<ref>Cheng, Maria, '[https://apnews.com/article/jet-lag-travel-sleep-1393330eaa4fe4f8edc2e0e8337daae4 How to avoid the worst of jet lag and maximize your travel time]'', Associated Press, August 21, 2024</ref>

The speed at which the body adjusts to a new rhythm depends on the individual as well as the direction of travel; some people may require several days to adjust to a new time zone, while others experience little disruption.

Crossing the [[International Date Line]] does not in itself contribute to jet lag, as the guide for calculating jet lag is the number of time zones crossed, with a maximum possible time difference of plus or minus 12 hours. If the absolute time difference between two locations is greater than 12 hours, one must subtract 24 from or add 24 to that number. For example, the time zone [[UTC+14]] will be at the same time of day as [[UTC−10]], though the former is one day ''ahead'' of the latter.

Jet lag is linked only to the distance travelled along the east–west axis. A ten-hour flight between Europe and southern Africa does not cause jet lag, as the direction of travel is primarily north–south. A four-hour flight between [[Miami, Florida]], and [[Phoenix, Arizona]], in the United States may result in jet lag, as the direction of travel is primarily east–west.

===Risk Factors===
Jet lag has a stronger impact when crossing more time zones over a few days. If someone has had jet lag before, then they are likely to have it again. Additional factors include arrival time, age, stress levels, sleep before travel, and use of caffeine or alcohol. Those over the age of 60 are more sensitive to circadian rhythm changes. The chances of jet lag increase with less sleep before a flight and higher stress levels.<ref>{{cite journal |last1=Herxheimer |first1=Andrew |title=Jet lag |journal=BMJ Clinical Evidence |date=29 April 2014 |volume=2014 2303|pmid=24780537|pmc=4006102 }}</ref>

Additionally, insufficient sleep before a flight can exacerbate jet lag symptoms. A well-rested state prior to travel helps the body adapt more efficiently to new time zones.<ref>{{cite journal |last1=Weingarten |first1=Jeremy A |title=Air Travel: Effects of Sleep Deprivation and Jet Lag |journal=Chest |date=2013 |volume=144 |issue=4 |pages=1394–1401 |doi=10.1378/chest.12-2963|pmid=24081353 }}</ref> 
High stress levels can also disrupt the body's natural rhythms, making it more difficult to adjust to a new time zone. Stress-induced hormonal changes may interfere with sleep quality and circadian alignment.<ref>{{cite journal |last1=Arendt |first1=J |title=Physiological Changes Underlying Jet Lag |journal=British Medical Journal (Clinical Research Ed.) |date=1982 |volume=284.6310 |issue=6310 |pages= 44–46 |doi=10.1136/bmj.284.6310.144|pmid=6275937 |pmc=1495508 }}</ref> 
Personal factors such as [[chronotype]] (morningness or eveningness preference), genetic predispositions, and overall health can affect how one experiences jet lag. For instance, individuals with a natural tendency to stay up late may find it easier to adjust to westward travel.<ref>{{cite journal |last1=Juda |first1=M |title=Chronotype Modulates Sleep Duration, Sleep Quality, and Social Jet Lag in Shift-Workers |journal=Journal of Biological Rhythms |date=2013 |volume=28 |issue=2 |pages=141–151 |doi=10.1177/0748730412475042|pmid=23606613 }}</ref>

===Double desynchronisation===
Double desynchronisation is the misalignment between: The body's internal clocks and the external environment (e.g., local time at your travel destination) and the body's central and peripheral circadian clocks (i.e., misalignment within different parts of your own body). 

There are two separate processes related to biological timing: [[circadian oscillator]]s and [[homeostasis]].<ref>{{cite journal |doi=10.1016/s1087-0792(98)90052-1 |title=Models of human sleep regulation |date=1998 |last1=Beersma |first1=Domien G.M. |journal=Sleep Medicine Reviews |volume=2 |issue=1 |pages=31–43 |pmid=15310511 |url=https://pure.rug.nl/ws/files/14639214/1998SleepMedRevBeersmaCorr.pdf }}</ref><ref name="dijk">{{cite journal |last1 = Dijk |first1 = D. J. |last2 = Lockley |first2 = S. W. |year = 2002 |title = Invited Review: Integration of human sleep–wake regulation and circadian rhythmicity |journal = Journal of Applied Physiology |volume = 92 |issue = 2 |pages = 852–862 |doi = 10.1152/japplphysiol.00924.2001 |pmid = 11796701 }}</ref> The master clock of the circadian system is located in the [[suprachiasmatic nucleus]] (SCN) in the [[hypothalamus]] of the brain. There are also peripheral oscillators in other tissues and organs, each having their own oscillatory rates that could be synchronized to the SCN oscillatory rate. The SCN's role is to send signals to the peripheral oscillators, which synchronise them for physiological functions. The SCN responds to light information sent from the [[retina]] and entrains its circadian rhythm to the external environment. It is hypothesised that peripheral oscillators respond to internal signals such as hormones, food intake, and "nervous stimuli" and may take longer time to synchronize to local time.<ref name="brown">{{cite book |last1 = Brown |first1 = S. A. |last2 = Azzi |first2 = A. |name-list-style = amp |year = 2013 |chapter =  Peripheral circadian oscillators in mammals |title = Circadian clocks  |pages =  45–66 |publisher = Springer |location =  Berlin}} {{cite book |author =  Center for Substance Abuse Treatment |title = Managing Depressive Symptoms in Substance Abuse Clients During Early Recovery |location = Rockville, MD |publisher = Substance Abuse and Mental Health Services Administration |year = 2008 |series = Treatment Improvement Protocol (TIP) Series, No. 48. |chapter = Appendix D: DSM-IV-TR Mood Disorders |chapter-url = https://www.ncbi.nlm.nih.gov/books/NBK64063/}}</ref>

The implication of independent internal clocks may explain some of the symptoms of jet lag. People who travel across several time zones can, within a few days, adapt their master clock SCN with light from the environment earlier. However, their [[skeletal muscle]]s, [[liver]], [[lung]]s, and other organs may adapt at different rates (peripheral circadian clocks may lag behind).<ref>{{cite journal |last1=Yamazaki |first1=Shin |last2=Numano |first2=Rika |last3=Abe |first3=Michikazu |last4=Hida |first4=Akiko |last5=Takahashi |first5=Ri-ichi |last6=Ueda |first6=Masatsugu |last7=Block |first7=Gene D. |last8=Sakaki |first8=Yoshiyuki |last9=Menaker |first9=Michael |last10=Tei |first10=Hajime |title=Resetting Central and Peripheral Circadian Oscillators in Transgenic Rats |journal=Science |date=28 April 2000 |volume=288 |issue=5466 |pages=682–685 |doi=10.1126/science.288.5466.682 |pmid=10784453 |bibcode=2000Sci...288..682Y }}</ref> This internal biological de-synchronisation is exacerbated as the body is not in sync with the environment{{snd}}a ''double desynchronisation'', which has implications for health and mood.<ref>{{cite journal |last1=Wirz-Justice |first1=Anna |title=Biological rhythm disturbances in mood disorders |journal=International Clinical Psychopharmacology |date=February 2006 |volume=21 |issue=Supplement 1 |pages=S11–S15 |doi=10.1097/01.yic.0000195660.37267.cf |pmid=16436934 }}</ref>

=== 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.|url-access=subscription }}</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.|url-access=subscription }}</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"/>