Although we don't believe in timing the market or panicking over daily movements, we do like to keep an eye on market changes -- just in case they're material to our investing thesis.

With another weekend in the rearview mirror and no tangible progress on resolving the debt ceiling issue, Wall Street flocked away from high-risk investments. This was bad news for equities, as more than 70% of stocks fell in value. The S&P 500 Index (SNPINDEX:^GSPC) fell 14 points, or 0.9%, to end at 1,676. 

Days like today are good times to remember the Wall Street adage to "buy under gunfire and sell under fireworks." It's easier said than done, of course, as shares of Regeneron Pharmaceuticals (NASDAQ:REGN) showed in their 3.1% slump Monday. The stock is up 80% in 2013, a heady gain that may have caused investors to take some chips off the table today, with the macroeconomic fears all over the headlines. It's a bit of a short-sighted reason to sell off, with potential blockbuster drugs targeting indications such as cholesterol, arthritis, and asthma in the pipeline. 

Big-box retailer Best Buy (NYSE:BBY) dropped 3% today. Holiday season, of course, is a retailer's best friend, but with debt-ceiling mumbo-jumbo dominating headlines, consumers might rather bury their cash in the backyard than spend it at Best Buy. That said, Best Buy isn't lacking for promotions to get foot traffic in stores: The electronics outlet is offering Apple's iPhone 5C for just $50 after a $50 gift-card subsidy. 

Lastly, New Jersey-based savings and loan Hudson City Bancorp (NASDAQ:HCBK) shed 2.8%. The stock, which values the company at $4.4 billion and is 44% more volatile than the broader market, is highly sensitive to secular bearishness on Wall Street, even though nothing materially changed with the company today. Investors will get a better pulse of the business on Oct. 21, when Hudson City's quarterly report hits the street.

Fool contributor John Divine owns shares of Apple. You can follow him on Twitter, @divinebizkid, and on Motley Fool CAPS, @TMFDivine.

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