Here you find a tutorial ("Analyzing cyclic patterns in psychological data_a tutorial") that explains how cyclic trends can be fitted to longitudinal data. In the file "Appendix" all R-code, which is used in this tutorial, is demonstrated and annotated. The R-functions that were developped for this tutorial make it easy to use cyclic models. The functions can be found in the "R-functions" folder. The data set used for the example (SmokingLapse) can also be found here.