Of American-English origin, the expression lightbulb moment denotes a moment of sudden realisation, enlightenment or inspiration.

This expression alludes to the representation of an illuminated light bulb above a character’s head in a cartoon or comic strip, indicating that this character has had an idea.

The earliest occurrences of the expression lightbulb moment that I have found are as follows, in chronological order:

1-: From Cavett (New York: Harcourt Brace Jovanovich, 1974), the autobiography of the U.S. entertainer Richard Alva Cavett (born 1936), written on the basis of dialogues with the U.S. cultural critic, editor of Time magazine and producer of The Dick Cavett Show James Christopher Porterfield (1937-2021):

It did strike me one day, like the light-bulb moment in a comic strip, that my magic act was the route to the top.

2-: From Kush’s gospel lights up eyes of the colts, by Rick Ostrow, published in The Sun (Baltimore, Maryland) of Monday 16^th August 1982:

There comes a time, usually early, in the regime of any boss when he must justify to his underlings the legitimacy of his command. Call it the “light bulb” moment, the instant when something goes on in the minds of those being bossed that says, “Hey, maybe this guy knows what he’s doing, after all.”
The moment is no less critical for the football coach than it is for the general, film director, chairman of the board or even the newspaper editor. If it doesn’t come eventually, discipline dissolves and the boss’s firing usually follows quickly.
The light bulb moment may well have occurred for Frank Kush during the second half of Saturday night’s come-from-behind, 19-to-14 victory over the New York Giants. Sure, it was only an exhibition game and did nothing to preclude on paper the possibility of a repeat of last year’s disastrous regular season, but it may have cemented in the minds of his players that the gospel according to Kush is reasonable. Get in shape, play hard and never stop hustling, says that gospel, and just see if good things don’t start happening out on that field.

3-: From Linguist draws fire left and right after chucking career for family: Goodbye work place, maternal instinct calls, by Deena Mirow, staff writer, published in The Plain Dealer (Cleveland, Ohio) of Tuesday 1^st October 1985:

[Deborah] Fallows quit her job as an administrator and research linguist at Georgetown University 5½ years ago so she could be a full-time mother to a son, 3, and soon-to-be-born second child.
“There really was no light-bulb moment,” recalled the author of “A Mother’s Work,” which explores ramifications of her decision and examines child care and other parenthood issues.
“I began to see things not in logistical terms, because those things were going well, but in psychological and emotional terms,” Fallow [sic] explained during an interview here. “I was losing my sense of serenity and calmness.”

4-: From the review of Defoe and the Idea of Fiction, 1713-1719 (Newark: University of Delaware Press, 1983), by Geoffrey M. Sill—review by Lennard J. Davis, published in The Eighteenth Century: Theory and Interpretation (Lubbock: Texas Tech University – Vol. 27, No. 2, Spring 1986):

Sill’s claim that Defoe was evolving toward fiction during the period he was writing ideological (hence fictional) pamphlets is not wrong, but it is undetermined. Sill discovers particular gropings toward fiction and illuminating, light-bulb moments in Defoe’s creative imagination, leading up to that crowning moment when Robinson Crusoe springs like Athena fully formed into the world of previously unnovelistic writing.

5-: From Teacher’s skill speaks for itself, by Dale Huffman, published in the Dayton Daily News and Journal Herald (Dayton, Ohio) of Wednesday 29^th October 1986:

Lynda Bragg […] has just been named the Speech Communication Association of Ohio’s Outstanding High School Speech Teacher of the Year.
[…]
In her acceptance speech for the state award, Mrs. Bragg said, “What really counts for me are those light-bulb moments when a student turns on to a skill or concept that has eluded him.”