But these moves are often short-lived. The S&P 500 averaged a return of 2.1 percent a month after a shutdown and trades positive 80 percent of the time.
"Anyone with capital markets experience knows that US government shutdowns are largely meaningless in the context of financial asset values," said Nicholas Colas, co-founder of DataTrek Research, in a note. "Getting sucked into a series of seemingly ominous headlines that smarter players already understand is a typical rookie trading mistake."
Investors also paid attention to the corporate earnings season. On Monday, Halliburton reported better-than-expected earnings, sending the company's shares up 3.7 percent.
The calendar fourth-quarter earnings season is off to a good start. As of Friday, 68 percent of the S&P 500 companies that had reported surpassed earnings expectations, while 85 percent of those companies had beaten sales estimates, according to FactSet.
What's more, "last week, 27 companies in the S&P 500 reported 4Q 2017 results, not one of those companies had its next quarter (i.e. 1Q 2018) EPS estimates lowered after releasing results," said Nick Raich, CEO of The Earnings Scout, in a note.
"This is an amazing stat because in EVERY earnings season over the last seven years, a majority of S&P 500 companies have had their next quarter estimates lowered after reporting. So, the fact not one of the companies reporting last week had EPS estimates lowered is remarkable," Raich said.
Netflix and TD Ameritrade are scheduled to report quarterly earnings after the bell Monday.
Elsewhere in corporate news, Twitter shares fell 1.4 percent after CNBC reported that executive Anthony Noto has been offered the CEO job at Social Finance.
—CNBC's Christina Wilkie contributed to this report.