Why does traveling east cause jet lag?

Traveling east causes jet lag primarily because it's harder for our bodies to adapt to going to bed earlier than to stay awake later. This difficulty arises from the asymmetric functioning of the autonomic nervous system, which is better at keeping us alert than it is at inducing sleep on demand. This means that when traveling east, you often need to sleep earlier according to the new time zone, which is more challenging than adjusting to a later bedtime when traveling west 1.