Jet lag is a temporary physiological condition that occurs when a person’s circadian rhythm is out of sync with the time zone they are in, and is a typical result from travelling rapidly across multiple time zones. For example, someone travelling from New York to London, i.e. from west to east, feels as if the time were five hours earlier than local time, and someone travelling from London to New York, i.e. from east to west, feels as if the time were five hours later than local time. More at Wikipedia