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.