Here is why, your body is tuned to what are called circadian rhythms which determine things like when you fall asleep, and when you wake up.
If you lived under a rock, your body's natural cycles would be slightly longer than 24 hours.
However, exposure to factors like daylight, temperature, and other people fine-tunes your body's cycles to your local time zone.
When you travel, this tuning is thrown out of whack, and it takes your body time to readjust.
This adjustment is easier if you're heading west because this lengthens your day in the natural direction of your internal clock.
But, if you're heading east, you're compressing your day, and working against your internal clock, which makes adjustment more difficult.
While none of this bodes well if you're a die-hard Dodgers fan, it may be worth remembering the next time you participate in the office pool.