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.

Most investors were pleased with today's modest gains in the stock market, with bullish investors pointing to solid consumer spending data on the economic front as supporting the generally positive sentiment in stocks. But holiday cheer didn't stop big losses at Tesaro (NASDAQ: TSRO), Steiner Leisure (NASDAQ: STNR), or Tower Semiconductor (NASDAQ:TSEM) today, with stock-specific news causing share prices to plunge.

Tesaro lost a quarter of its value after phase 3 trial results for its chemotherapy-side-effect treatment rolapitant left investors wanting more from the company. Rolapitant achieved the desired primary outcome in the study, but it failed to meet secondary outcomes with sufficient statistical significance. With the company enrolling patients in a third phase 3 trial, Tesaro nevertheless hopes to apply for FDA approval of the drug, but investors aren't nearly as certain about the drug's prospects after the study's failures.

Steiner Leisure dropped 15% after the company announced that Royal Caribbean Cruises' (NYSE:RCL) Celebrity Cruises unit would not renew its agreement with Steiner after it expires at the end of the year. For Steiner, the move comes as a distinct loss, although the spa, salon, and health-club operator has cruise operations on 155 ships across a wide span of different carriers, as well as many land-based spa operations located hotels from many well-known chains. Steiner said that the agreement could reduce earnings by $0.24 per share, about 7% of its trailing earnings, which makes today's plunge seem oversized by comparison.

Tower Semiconductor gave back 8% as investors appeared to take profits after a more than 50% jump for the stock on Friday. The jump last week came after Tower said that it would buy three semiconductor production facilities from Panasonic, creating a joint venture in which the Israeli semiconductor company's Japanese unit will own a controlling 51% interest and Panasonic will retain 49%. Investors hope that Tower's move will help it take advantage of improving conditions in the semiconductor industry, especially as demand for chips appears finally to be on the rise after a long period of sluggishness in tech buying. Pullbacks like this might seem significant, but after such a big jump, it's hard to take a relatively small move as anything more serious.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.