There is a distinct biological transition when a New Zealand summer sets in, the call of Tūī in the coastal pōhutukawa, the hum of cicadas, and the sudden shock of cold ocean water along the Browns Bay shoreline. After months of winter strain and end-of-year rush, summer feels like a welcome break.

Look closely at human biology, and you will find that almost every physiological process is rhythmic. Our cells, organs, and nervous systems do not operate at a constant, linear output. They rely on complex biological clocks, from the millisecond firing of heart rate variability (HRV) and the 24-hour circadian rhythm to monthly infradian hormonal cycles.