How about everyone knowing what the word 'nocturnal' means, but they can't think of the word describing the opposite behaviour and resorting to using the word 'normal'.

nocturnal - Done, occurring, or active at night. [0]

diurnal - 1. Of or during the day. 2. Daily; of each day. [1]

crepuscular - Resembling or relating to twilight. [2]

[0] https://en.oxforddictionaries.com/definition/nocturnal [1] https://en.oxforddictionaries.com/definition/diurnal [2] https://en.oxforddictionaries.com/definition/crepuscular