A Neural Calendar in the Human Brain
The suprachiasmatic nucleus (SCN) of the hypothalamus is the principal component of the mammalian biological clock, the neural timing system that generates and coordinates a broad spectrum of physiological, endocrine and behavioral circadian rhythms. The pacemaker of the SCN oscillates with a near 24-hour period and is entrained to the diurnal light-dark cycle. Consistent with its role in circadian timing, investigations in rodents and non-human primates furthermore suggest that the biological clock is also the locus of the brain's endogenous calendar, enabling organisms to anticipate seasonal environmental changes.
Although human beings are not considered to be particularly photoperiodic many of the seasonal cycles of human biology are under environmental control and seem to be driven by an endogenous clock synchronized with the seasons. Whether humans indeed have a seasonal timekeeper and where it is located is unknown. It could well be that the neuronal changes and adaptations in humans that occur in response to a seasonally changing environment are driven by an endogenous circannual clock located in the SCN, and that this neural calendar is reset by the seasonal fluctuations in photoperiod.