The impact this continuous body cal

The impact this continuous body calendar has on some species' physiology and behaviour is more surprising than you might think. The annual shortening and lengthening of daylight even causes a change in the eye colour of reindeer, from gold to blue. Gold is better during the endless daylight of summer, while blue is more light-sensitive, which helps the reindeer see more in the gloom of winter. The reindeer's internal calendar also leads to a Seasonal shift in melatonin. In summer, when there's much more food around, melatonin production is suppressed so the reindeer doesn't feel tired, meaning it can graze almost continuously.