There are two main aspects of modern life that can cause a disruption to our biological rhythms, shift work and jet lag.
This is the term to refer to the physiological effects of disrupting circadian rhythms; however it most commonly used to describe the disruption we experience from flying. It occurs because we cannot cope with the change from moving to different time zones. For example, your body could be used to British time, but the exogenous zeitgebers have changed dramatically because of the new time zone. It is believed that for each time zone you travel across it will take a day for your body to adjust.
Other factors – There are other factors that could compound the feeling of jet lag, these include the stress of flying, the constant noise, the pressurized air in the cabin etc.