Shares of Vanda Pharmaceuticals (NASDAQ:VNDA) have fallen more than 20% since Thursday, even though the company said that its phase 3 clinical trial met its primary goal.

How could that be? Investors only care about how much of the drug Vanda can sell, and study results that may be good enough for continued clinical testing and possible marketing approval don't guarantee a commercially successful drug.

What has discouraged investors is that while Vanda's potential insomnia treatment, tasimelteon, met its primary goal of improving patients' ability to fall asleep (called sleep onset), it produced mixed results at best for some of its secondary goals that had to do with whether the drug improved patients' ability to stay asleep (called sleep maintenance).

Tasimelteon still could reach the market even if Vanda only makes sleep-onset claims on the drug's label, but improving insomniacs' sleep maintenance is where the money is, and what has made compounds like Sanofi-Aventis' (NYSE:SNY) Ambien and Sepracor's (NASDAQ:SEPR) Lunesta such sales successes. Rozerem, which is sold by Takeda, is somewhat similar to taximelteon and has had limited commercial success since its launch in 2005. It, too, lacks these sleep-maintenance claims on its label, and had sales of just $111 million for its latest fiscal year.

Vanda has other development problems not specific to tasimelteon, but which could affect all insomnia drugs in development going forward. Last year, the Food and Drug Administration handed a harsh rebuke to Neurocrine Biosciences (NASDAQ:NBIX) after reportedly requesting a huge, lengthy, and impractical amount of extra clinical trial work before it would approve insomnia treatment indiplon. If the FDA has similar requirements for tasimelteon, then Vanda, too, will have a hard time getting it to market, regardless of whether it can make sleep maintenance claims on the label or not.