Why is eastward travel from New York to Paris functionally more difficult for the internal clock?

The internal clock must rush forward, and the resetting mechanism is not perfectly symmetrical across inputs.

The adjustment required for jet lag illustrates a functional consequence of the discovered molecular clockwork. When traveling eastward, for example from New York to Paris, a person effectively 'loses' time, meaning the internal biological clock must rapidly advance or speed up to align with the new local time zone. This adjustment is often noted as being more difficult or requiring more time than traveling westward. The reason this asymmetry exists lies in the nature of the molecular feedback loop discovered by Hall, Rosbash, and Young. The mechanisms the clock uses to advance its phase (speed up) versus the mechanisms it uses to delay its phase (slow down) are not equally efficient or responsive to the environmental cues like light exposure, leading to a functional difference in adjustment speed depending on the direction of travel.