Cicada and circadian are easily confused words.
The spell-check application of most word processing software programs would not catch a slip-up of these two words. Spell-check is looking for words that aren’t in its dictionary, and words that resemble words in its dictionary but are possibly spelled wrong. Spell-check isn’t perfect. It doesn’t know and can’t guess what word you wanted or what word you meant, it can only judge the words on the page. If you used words that are all spelled correctly, it gives you a pass anyway.
Autocorrect suggests words that start with the same letters. It’s suggesting what word you may want to save time, but quite often, its suggestions are pretty off base. They don’t help you out, but they do make you laugh.
Cicada (pronounced “si-kay-duh”) is a noun. It means a very large flying insect with a trademark hissing call that it emits during the morning and evening hours of summer in warm climates.
Circadian (pronounced “sir-kay-dee-ann”) is an adjective. It means the biological cycles that repeat every 24 hours. The circadian rhythm is the sleep and wake cycle the human body uses to keep itself running.
The following story uses both words correctly:
Circe, originally from Greece, was spending her summer in the eastern US. The time zone changes had messed with her Circadian rhythm. The loud buzz of cicadas outside, an unfamiliar noise, was also making it hard to get rest in a new country.