Entrainment refers to an individual's chronobiological, physical, and behavioral relationship with their environment. Specifically, this refers to an individual's physical and mental adaptation to the time zone in which they live. This includes the human circadian rhythm (24 hour cycle) that conditions the individual when to sleep and wake-up, when to eat, etc. Traveling upsets this entrainment and it takes a few days for the body to adapt to the new region and reset the circadian rhythm of the body. This is why traveling to other parts of the world (such as a European tour) creates such exhaustion.