Investors are using options to brace for big swings this week as Wall Street enters the peak of the most volatile month for stocks historically.

Options on the CBOE Volatility Index, Wall Street’s so-called fear gauge, were one of the top-traded contracts in the options market as investors made bets on a sharp jump in the index.

This week “is the real start of a month to be nervous about,” said Brian Overby, a senior options analyst at onlinebrokerage TradeKing in Charlotte, North Carolina.

“Especially, because the volatility has come off so much,there is a lot of complacency in the market. So if we get one(item of) bad news, that will cause a big jump.”

September is typically a weak month for stocks, and volatility reaches its peak as traders are fully back to work from summer holidays.

The largest open interest on VIX options was on the September $45 calls, suggesting some investors were betting on the gauge to double the current level by this week’s expiration, said Ryan Detrick, a senior technical analyst at Schaeffers Investment Research in Cincinnati, Ohio.

The index usually has an inverse relationship with the Standard & Poor’s 500 benchmark as it tracks option prices that investors are willing to pay as a protection on the underlying stocks.

The Dow and S&P 500 both closed last week with a seventh gain in eight sessions in a turnaround period for stocks that has seen investors’ worst fears about the economy start to dissipate.

But the gains were made on the second-lightest trading volume of the year so far as investors remained on guard for more deterioration in the market.

The week’s economic calendar includes retail sales due tomorrow, industrial production and capacity utilisation on Wednesday, the Producer Price Index and jobless claims on Thursday and then the Consumer Price Index and University of Michigan/Thomson Reuters consumer confidence on Friday.