UPDATE 1-FDA holds up Cephalon's Nuvigil jet lag approval

* FDA questions robustness of data

* Cephalon to meet with FDA to discuss issues

* Shares fall almost five percent

NEW YORK, March 29 (Reuters) - U.S. health regulators declined to approve Cephalon's CEPH.O sleep disorder drug Nuvigil as a treatment for jet lag, questioning the quality of some of the data used to seek approval, and the drugmaker's shares fell nearly 5 percent.

Nuvigil, a follow-up to Cephalon’s flagship sleep disorder drug Provigil, is approved to treat sleep apnea, narcolepsy and shift work sleep disorder.

The company was seeking to expand the drug’s label to include approval to treat excessive sleepiness associated with jet lag resulting from eastbound travel.

Instead of a final decision from the U.S. Food and Drug Administration (FDA), Cephalon received a so-called complete response letter indicating that the agency would not approve the drug for jet lag at this time.

Cephalon said Nuvigil met the main goals of pivotal studies comparing the drug with a placebo, adding that it had worked closely with the FDA to design the clinical trials to evaluate Nuvigil for eastbound jet lag.

“Although we reached statistical significance on both primary endpoints, the Complete Response letter raised questions regarding the robustness of the PGI-S data (Patient Global Impression of Severity),” Cephalon’s Chief Medical Officer Lesley Russell said in a statement.

“We have already reviewed this issue with the FDA and will be scheduling a meeting with the agency in the near future to discuss it further,” Russell said.

There was no indication about how long an approval for the jet lag indication might be held up.

Cephalon shares fell almost 5 percent to $69 in extended trading from their Nasdaq close at $72.62. (Reporting by Bill Berkrot)