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

What: Shares of senior-living services operator Sunrise Senior Living (NYSE: SRZ) have fallen and can't get up, losing as much as 10.4% of their value on moderate-volume intraday trading today.

So what: The drop comes on a downgrade by analyst firm Stifel Nicolaus. After more than doubling over the past three months and tripling in six, Sunrise had reached Stifel's $12 target price and was due for a reassessment.

Now what: Sunrise still looks very affordable next to higher-priced rivals Assisted Living Concepts (NYSE: ALC) and Capital Senior Living (NYSE: CSU), but there's nothing wrong with taking a step back and redoing some homework on your holdings. In Sunrise's case, the 8.3% short-sale troupe may be onto something, as the company's operating margins look absolutely anemic next to its peers', and last quarter's terrific numbers rode in on a truckload of one-time revenue gains.

Interested in more info on Sunrise Senior Living? Add it to your watchlist.

Fool contributor Anders Bylund holds no position in any of the companies discussed here. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool is investors writing for investors.