Presentiment – circadian clocks giving plants and animals a sense of time

Presentiment is that long shadow on the lawn
Indicative that suns go down;
The notice to the startled grass
That darkness is about to pass.

Emily Dickinson

Sometimes you find in literature beautiful expressions of technical terms that are otherwise dry and stuffy. Presentiment, by Emily Dickinson, is one of those beautiful expressions. Why did she decide to write a few words about twilight, and at the same time so succinctly summarise one of the key features of the circadian clock? Apparently Dickinson spent much of her adult life withdrawn from the world and, in doing so, she was probably in a position to watch and notice the hidden-in-plain-sight details of the world, such as how the length of shadows allow you to approximate the time of day and how grass may tell time without watches.

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