Cosinor analysis uses the least squares method to fit a sine wave to a time series. Cosinor analysis is often used in the analysis of biologic time series that demonstrate predictible rhythms. You supply the p-value (alpha) to get the corresponding confidence limits. The ‘confidence interval’ for any parameter is the probability that the true value of the parameter will be found within an interval defined by your chosen p value. The confidence interval tell the investigator if there is or is not a circadian rhythm present at the chosen probability, ‘alpha’. You can also decide the acceptable confidence limits :)