Source: Castlight Health,

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 in Castlight Health (NYSE:CSLT) plummeted by more than 25% earlier today after reporting better than expected fourth quarter sales, but guiding for less-than-expected first quarter and full year revenue.

So what: Castlight Health offers services that help employees and health insurance members make better, cost-saving health care decisions. Its products provide patients and members with quality and cost data that they can use to find the right healthcare at the right price.

Although that's an intriguing business model, investors appear to have gotten pretty far out in front of their skis on this one. Following its IPO last March, investors bid up shares so high that despite revenue being measured in the tens of millions of dollars, the company was sporting a market cap of more than $2.5 billion.

For the fourth quarter, Castlight reports that its sales totaled $14.48 million, up 182% year-over-year, and that it lost -$0.17 in the quarter, which was $0.05 better than Wall Street forecasts. Although those fourth quarter results were solid, the company also issued disappointing guidance.

In the first quarter, Castlight expects that its revenue will be between $15.3 million and $15.6 million, and for the full year, the company believes that its sales will reach between $74 million and $77 million. Those forecasts are shy of analysts' $16.1 million and $81.2 million consensus estimates, respectively.

Now what: Castlight's premise is a good one, but it relies heavily on data feeds on quality and price from insurers including Aetna and Anthem. If those companies ever decide to shut off Castlight's feed, it could create a big problem for the company down the line. For now, that risk hasn't dented Castlight's growth. Castlight's sales increased by 252% in 2014 and its forecast -- while below estimates -- still reflects year-over-year growth of 62% (at the low end of guidance) in 2015. Although that growth is enticing, the fact that the company expects to lose money again this year is reason enough for me to remain on the sidelines on this stock.