Hormones, subjective night and season of the year

Production and release of many mammalian hormones exhibit circadian rhythms controlled by a pacemaker located in the suprachiasmatic nuclei (SCN) of t he hypothalamus. Under conditions when the circadian pacemaker free-runs wi th a period close to, but not equal to 24 h, subjective day and night may n ot be identical with the environmental day and night. The present study was aimed to define the phase and state of the circadian pacemaker when the ci rcadian system is experiencing subjective night and to ascertain whether an d how such a defined subjective night depends on the photoperiod. The resul ts indicate that the subjective night may be defined as the time interval w hen i) light stimuli can reset the circadian system, ii) pineal melatonin p roduction and photic induction of the c-Fos gene in the ventrolateral SCN a re high, and iii) the spontaneous c-Fos protein production in the dorsomedi al SCN is low. Such a defined subjective night and, logically, the whole ci rcadian pacemaking system depend on the photoperiod and hence on the season of the year which the animals are experiencing.