The body temperature of 2 persons closed off to the outside world was pergressively analyzed with a short analyzing interval showing that the spontaneous and controlling rhythms of T = 30 h are present as previously demonstrated by ASCHOFF and BLUME. The characteristic properties and differences of both curves are described. Especially it is demonstrated that a transient process is present in the first curve, but not in the second curve. The presence of both rhythms is confirmed by the pergressive Fourier analysis of the first curve with a long analyzing interval. The problem to find out correct values for the parameters of hidden rhythms with the aid of the amplitude spectrum is pointed out. It is not present for the pergressive Fourier analysis.