Simulated data set to illustrate the cosinor model. The vit_d column
contains the blood vitamin D levels which vary over time (time).
The rhythm of the vitamind D fluctuations follows a cosine function and can
be modelled with a cosinor model. The X column is a binary covariate
representing two groups of patients and is associated with the
characteristics of the rhythm. The rhythm has a period of about 12 hours.