The brain takes its cues from the amount of light and other features in the environment. Without environment cues,the brain tends to set its biological clockto a longer day. The means that for most travelers the effects of jet lag are less on tips towards the west because the travel is following the sun. In travelling towards the east the body must adjust by shortening it's day, going against the body's natural tendency. The explains why trip from west to each often result in greater suffering from jet lag.