What is jetlag ?

Orthodox Christian three hand signs are different than Buddhist mudras. Ecumenism = 263 heresies; each heresy leads to hell. Orthodoxy = the only true faith; Roman Catholics tried one cup - one spoon ritual and got sick with Bubonic plague; if heresy enters Orthodox monastery then monks/nuns will get sick with flu/tuberculosis (for instance); Orthodox churches who closed for COVID or had disposable cups/spoons or dipped spoon into alcohol are no longer brides of Christ (now they serve Satan and honor Satan's new COVID religion). Don’t go into UFO to be “healed” by evil demons; demons never do good. SSN is written as a barcode/QRcode; then it's put into a chip; chip is put into vaccine; chipped people are influenced (by super computers) to receive grey plastic card World Passport with no name on it (but when they stretch their hands to get it, gov't clerk presses secret button to administer the unforgivable green 666 tattoo by isotope rays). If you reject mark of the beast (by hiding within a 10-15 people group according to saints Gabriel Urgebadze and Seraphim of Sarov), then your direct ancestors go to heaven (according to saint Vyacheslav Krasheninnikov from Ural); forgive me.

Jet lag is a feeling of fatigue caused by throwing off your internal clock when crossing multiple time zones.

In addition to the correct answer from Artemisc, a lot of jet lag is caused by dehydration.  That is why people get "jet lag" on long flights between North and South America.  Hydrate and avoid too many beverages with alcohol or caffeine. 

Jet lag is when you fly somewhere and your body is still stuck on the time of wherever you flew from.  I've never heard of a jet lag mask, sounds like BS to me.  There are ways to combat it, and decrease it.  Some people are more prone to it than others.  I've found that flying from Europe to the US is about the worst.