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Liberty Science Center's Genius Gala 4.0
Amazon founder Jeff Bezos speaks during Genius Gala 4.0 at Liberty Science Center on May 1, 2015 in Jersey City, New Jersey.
Taylor Hill—Getty Images founder and Chief Executive Officer Jeff Bezos got a lot richer while he slept on Thursday night, after the company reported its quarterly earnings and its shares rose 20.1%.

The stock jump added $8 billion to Bezos' net worth as the market opened on Friday, but by the time the market closed, he had given back roughly half of that gain. The shares, which had briefly hit a record high of $580.57 overnight, closed at $529.42.

It is all in a day's work - or a night's sleep - for Bezos, who owned 83.92 million shares, or roughly 18% of Amazon's outstanding stock of May 14.

His stake easily dwarfs those of other insiders. Diego Piacentini, senior vice president of international consumer business, is the second-largest insider holder behind Bezos, with 110,000 shares. Jeffrey Wilke, senior VP of consumer business, owns 80,000 shares.

The stake held by Bezos would hold a market cap of about $48.6 billion on its own, which is bigger than S&P 500 names such as FedEx, Caterpillar and Netflix. It is more than 18 times more than the smallest company in the S&P 500 by market cap, Joy Global.

It is too soon to tell whether Bezos was up overnight selling some of his highest-priced shares, but an examination of his filings with the U.S. Securities and Exchange Commission show he has been slowly cutting his stake, mainly by donating stock to charity. He owns roughly 5% fewer shares now than his 88.1 million as of Aug. 1, 2011.

Amazon declined to comment.

Update: This story was updated to reflect the closing stock price for Amazon on Friday, July 24.