The quality or state of being diurnal; the characteristic of occurring or being active during the daytime.
Formed by adding '-ness' to 'diurnal,' this abstract noun creation follows standard English word-building patterns that became especially common in the 18th-19th centuries.
While 'diurnalness' is a technically valid English word, most people would just say 'daytime activity' or 'being diurnal.' It shows how English lets you theoretically turn any adjective into an abstract noun, even if the result sounds clunky!
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