Cosinor analysis of IOP data of one subject. For this subject, six annual 24-h IOP profiles, each consisting of at least 6 measurements, were available for analysis. A cosine curve was fitted to each series. The cosine curve fitted to the first 24-h series, shown on the left side, fits very well, whereas the cosine curve for series number 4, on the right side, exhibits only a mediocre fit. The mean IOP level estimated by the cosine model, called MESOR, is similar in both cases (between 16 and 17 mmHg). The middle of the figure shows a clock plot with the time of day at which the curves fitted to each 24-h series peaked (i.e. the acrophase). All acrophases are plotted on the unit circle. Although it may be interesting in certain situations to use the distance to the center to symbolize the amplitudes of the cosine curves, it is not useful in this case, because it prevents the calculation of the overall direction of acrophases (i.e. mean acrophase). Circular data warrant special mathematical analysis: to calculate the overall direction of acrophases, the X- and Y- coordinates of the points shown in the clock plot are averaged separately. The vector formed by connecting the origin and the point defined by the mean X- and mean Y-coordinate indicates not only the mean direction of acrophases but also the dispersion of acrophases around the circle. A long vector signifies a small dispersion or a high stability in phase timing. The Rayleigh test is used to test the significance of the overall distribution, being significant when it is highly unlikely that acrophases are equally likely any time of day (i.e. uniformly distributed).