The Jetlag Calculator
“Jetlag is actually caused by disruption of your ‘body clock’, a small cluster of brain cells that controls the timing of biological functions (circadian rhythms), including when you eat and sleep. The body clock is designed for a regular rhythm of daylight and darkness, so it’s thrown out of sync when it experiences daylight and darkness at the ‘wrong’ times in a new time zone. The symptoms of jetlag often persist for days as the internal body clock slowly adjusts to the new time zone.”