60 Hours a week is the sweet spot of productivity?

On Wednesday, Sergey Brin, co-founder of Google, said the company could lead the industry in artificial general intelligence — when machines match or become smarter than humans — if employees worked harder.

“I recommend being in the office at least every weekday,” he wrote in a memo posted internally on Wednesday evening that was viewed by The New York Times. He added that “60 hours a week is the sweet spot of productivity” in the message to employees who work on Gemini, Google’s lineup of A.I. models and apps.

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Does anyone know where he got this number from? I've asked a few people and no one seems to know.

I assume he's talking about 12 hour work days as well not like 8 hours for 7 days essentially having no time off at all