Seasonal variations

Abstract Throughout the chapters of this textbook on biological rhythms, a spectrum of rhythm periods has been examined, ranging from the ultradian stages of sleep, through the circadian fluctuations in physiological and performance variables, to the infradian rhythm of the menstrual cycle. In this chapter, biological rhythms with even longer periods than the circamensal cycle are examined − those that recur once per year. Such rhythms may be defined as ‘circannual’, which denotes a period of about 1 year ± 2 months, or ‘seasonal’. Strictly speaking, seasonal changes are brought about by changes in environmental fluctuations and are not observed in the absence of such changes, whereas circannual rhythms may be endogenous and/or desynchronized from the calendar year.