What exactly is jet lag?

Jet lag, also jetlag or jet-lag, is a physiological condition which is a consequence of alterations to the circadian rhythm. Such alterations result from shift work, daylight saving time, altered day length, or as the name implies, transmeridian travel as on a jet plane. They are known as desynchronosis, dysrhythmia, dyschrony, jet lag, or jet syndrome. The condition is generally believed to be the result of disruption of the "light/dark" cycle that entrains the body's circadian rhythm. It can be exacerbated by environmental factors.

being drowsy or having nausea...

When you travel by air and go from one time zone to another, your body has to get accustomed to the new time zone hours.

Jet lag is when you are tired after a long flight.

When you go on a long flight and aren't used to it...It takes a while to get used to the time difference and for the first while (depends how often you do it) it's odd.

Jet lag, also jetlag or jet-lag, is a physiological condition which is a consequence of alterations to the circadian rhythm. Such alterations result from shift work, daylight saving time, altered day length, or as the name implies, transmeridian travel as on a jet plane. They are known as desynchronosis, dysrhythmia, dyschrony, jet lag, or jet syndrome. The condition is generally believed to be the result of disruption of the "light/dark" cycle that entrains the body's circadian rhythm. It can be exacerbated by environmental factors.

It is a combination of things -- including the adjustment to a new reality of space and time -- our bodies have their own internal clock and have to reset. Some people also experience a fuzzy headed feeling after flying a distance (maybe it is from having to remain in the enclosed space for so long or from being so inactive for a period of time).

Due to differences in time zones, jet lag is when a person travels from one time zone to another, ex. Pacific time zone - Eastern time zone, and their sleeping patterns or daily activities are mixed up because of the difference in time (here 3 hours). More commonly used to refer to international flights.

jet lag is a way of saying people out of sync with the locality they are in

Theories on jet lag are numerous the "most reasonable one" imo is that the body regulates the release of chemicals within our body for optimum operation. The thing is though that the release and effects of the chemicals is not "instantaneous" it is like a factory.

You need to do a b c d e f g etc.. however each step is timed to happen at the best posible time.

The main issue with our "biorythms" is that they can get thrown out of sync by a number of natural and artifical events, so too can they be put back into sync by the same process.

The food we eat can effect our biorythms
The Electric screens we look at can effect our biorythms (in part by visual signals received by our optics signals.. even the effect of the photons when they are converted into electrical signals in our brain, those signals then effect our cognitive process as stimulus, as well as resonant energy flow, these stimulants affect what chemicals are release and when both psychologically and pysiologicallally.

Now when we fly somewhere else, our body can be sent to a profoundly different environment, and our biorythms arn't attuned to this.. daylight could be occuring at a different time then planned for, the temperature could be different, the elevation, the magnetic field strenght and countless other things, essentially our body isn't prepared for this. While "we think" that we are in one environment, our body is prepared for another environment. This causes symptoms such as fatigue when various chemicals times for release that let us feel tired or awake are released from their factories.

It is interesting to note that light therapy can be used to "realign" our bodies in a fast period of time, much as the absence of a colour can aid in altering our likely reactions due to the latent interpretation of that photon frequency by our brains stimulus response system.

It is largely light reception effecting biorythms but many other factors are involved.

Its basically being tired from sitting on a plane for a long time, or most of the day is getting on and off of planes. It just wipes you out. AND youre thousands of feet in the air, it effects our heads.