A simple technique is described that enables the degree of adaptation at different times of day to be studied in organisms that have been subjected to a sudden change in the Zeitgeber (as typically found in shiftwork or after transmeridian flight). The investigator is thus able to place greater emphasis on some readings than on others and to measure the amount of disruption at a particular time of day (e.g. for an important meeting) rather than to merely rely upon an overall daily measure. The method uses the likelihood ratio statistics which also allows a powerful test to be made of the statistical significance of any apparent adaptation. The method is illustrated on some 'jet-lag' data, where it was able to determine differential rates of adaptation according to the time of day at which the subjects were tested.