<unk>  is a chronobiological problem, similar to issues often induced by shift work and the circadian rhythm sleep disorders. When travelling across a number of time zones, the 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 has grown accustomed. The body's natural pattern is upset, as the rhythms that dictate times for eating, sleeping, hormone regulation, body temperature variations, 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<unk> ged. The speed at which the body adjusts to the new schedule 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<unk> , as the guide for calculating<unk>  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.<unk>  is linked only to the trans-meridian (west–east or east–west) distance travelled. A ten-hour flight between Europe and southern Africa does not cause<unk> , 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<unk> , as the direction of travel is primarily east–west. Double desynchronisation There are two separate processes related to biological timing: circadian oscillators and homeostasis. The circadian system is located in the suprachiasmatic nucleus (SCN) in the hypothalamus of the brain. The other process is homeostatic sleep propensity, which is a function of the amount of time elapsed since the last adequate sleep episode. The human body has a master clock in the SCN and also peripheral oscillators in tissues. The SCN's role is to send signals to peripheral oscillators, which synchronise them for physiological functions. The SCN responds to light information sent from the retina. It is hypothesised that peripheral oscillators respond to internal signals such as hormones, food intake, and "nervous stimuli". The implication of independent internal clocks may explain some of the symptoms of<unk> . People who travel across several time zones can, within a few days, adapt their sleep–wake cycles with light from the environment. However, their skeletal muscles, liver, lungs, and other organs will adapt at different rates. This internal biological de-synchronisation is exacerbated as the body is not in sync with the environment – a "double desynchronisation", which has implications for health and mood. Delayed sleep phase disorder Delayed sleep phase disorder is a medical disorder characterized by delayed sleeping time and a proportionately delayed waking time due to a phase delay in the endogenous biological master clock. Specific genotypes underlie this disorder. If allowed to sleep as dictated by their endogenous clock these individuals do not suffer any ill effects as a result of their phase shifted sleeping time.
Jet lag
Lid lag
Mood swing
Moonrise 
Speech delay
Pathological lying
Far-sightedness
Pixl