Wall Street, however, has calmed down. And it was not so panicked in the first place. During the recent turbulence, the VIX, a fear gauge, hit only its 24th-highest monthly level in the past 35 years. The index measures expected volatility for American stocks, based on the price investors pay for insurance.

Wall Street's main indexes opened lower on Wednesday, as investors awaited the outcome of a pivotal debate around U.S. President Donald Trump's tax-cut bill that has fanned concerns about the country's growing debt.