The stock jump added $8 billion to Bezos' net worth as the market opened on Friday, but by midafternoon, he had given back $3.1 billion of that gain. The shares, which had briefly hit a record high of $580.57 overnight, stabilized near $540.

It is all in a day's work - or a night's sleep - for Bezos, who owned 83.92 million shares, or roughly 18 percent 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 Corp, Caterpillar Inc and Netflix Inc. It is more than 18 times more than the smallest company in the S&P 500 by market cap, Joy Global Inc.

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 percent fewer shares now than his 88.1 million as of Aug. 1, 2011.

Amazon declined to comment.