Nasdaq’s late-night announcement of plans to rebalance its benchmark Nasdaq-100 Index spurred volatility in Apple shares and surprised U.S. fund managers who questioned the timing of the announcement.

Rebalancing is not an unusual event, but the Nasdaq announced it at around 3 a.m. on Tuesday, making early trading more volatile.

“It’s surprising and unsettling that the Nasdaq would release this information in the middle of the night, let alone at this moment in the calendar — right ahead of earnings season,” said Keith Wirtz, president and chief investment officer at Fifth Third Asset Management in Cincinnati, Ohio.

Nasdaq OMXsaid it will cut the weight of 82 securities to bring them more in line with their market capitalizations.

By far, Apple will be the most affected by the move, with its weighting slashed to 12.3 percent from 20.5 percent, though it will remain the largest component of the index. Apple has a market cap of roughly $314 billion.

The news initially knocked down shares of Apple while it boosted shares of Microsoft Corp , whose weighting will increase, and was one reason for a fall in Nasdaq futures before the opening bell.

Apple shares fell around 3 percent in premarket trading but by late afternoon had reversed much of its early losses to trade down 0.7 percent at $338.92.

“This is just a temporary blip. It won’t impact the long-term story of the stock,” said Jerome Heppelmann, portfolio manager and chief investment officer at Old Mutual Focused Fund.

John Jacobs, executive vice president in Nasdaq’s global index group, said the timing of the Nasdaq was in order to be ahead of the market open in Europe where the Nasdaq-100 Trust exchange-traded fund also trades. Nasdaq-100 futures trade around the clock on CME Globex.

Peter Kenny, managing director at Knight Equity Markets in Jersey City, New Jersey, said the market was taken by surprise, nonetheless.

“It definitely was not expected and I don’t think it was in any way, shape or form priced into the market,” he said. ”Some people might have suspected a move of this magnitude might have been telegraphed a little more intentionally.”

“Clearly, some people think the move might well have been a bit more transparent.”

More than $330 billion in funds and $40 billion in exchange-traded funds are benchmarked to the Nasdaq 100, including the QQQ, the most actively traded Nasdaq-tracking ETF, which has a market capitalization of $24.4 billion.

“In the short term, this is going to have a significant impact and create additional volatility,” said Oliver Pursche, president of Gary Goldberg Financial Services in Suffern, New York.

The rebalancing goes into effect May 2. It will affect the relative weights of all the securities in the index and cause popular index-tracking funds such as the PowerShares QQQ to buy and sell shares to match the new composition.

Much of Apple’s excess weighting was distributed between other large-cap technology shares that had become underweighted relative to Apple.

Microsoft’s weighting will rise 4.9 percentage points. Its

stock rose 1.1 percent to $25.84.

Other big changes include Oracle Corp, whose weighting will rise to 6.7 percent from 3.3 percent, Intel Corp, up to 4.2 percent from 1.8 percent, and Cisco Systems up to 3.7 percent from 1.6 percent.

“You’ll see additional volatility from this news, especially given that the Russell Index will also go through its reconstitution in May and June,” Wirtz said.

James Angel, a Georgetown University professor who specializes in financial market regulation and sits on the board of directors of the Direct Edge stock exchange, said there should be advance notice of a planned index change.

However, he said Nasdaq might be trying to avoid some of the gaming that has occurred in the rebalancing of the Russell 2000 by giving less advance notice.

“If all they’re doing is following their formula, then the people who need to know presumably know the formula,” Angel said. “On the other hand, if they’re doing something that they have the discretion to do but isn’t exactly the formula, then it would be useful to give people a little heads up.”

The relative positions of the issues within the index will remain the same once the rebalancing takes effect May 2, according to Nasdaq. Apple will remain the largest component.

The rebalancing will be based on index securities and shares outstanding as of March 31, the exchange said.