Circadian rhythms just means daily rhythms, which can be affected by many factors apart from the obvious time zone change.American police forces discovered that when they rostered plods for 4 months of earlies followed by 4 months of days, then 4 months of nights, more crimes were solved, less sick days were taken and the accident rate decreased significantly. Pilots rosters are so random that you never get a chance to adjust to a particular daily cycle, apart from the night freight boys.
Unfortunately the police model would be unworkable in an airline environment, not to mention unacceptable by most pilots.