(Reuters) - Transcept Pharmaceuticals Inc said the U.S. health regulator has approved its sleep drug Intermezzo as a first-of-its kind treatment for a particular type of insomnia.

Intermezzo, chemically known as zolpidem tartrate sublingual, can now be used as a therapy for insomnia characterized by middle-of-the-night waking followed by difficulty returning to sleep.

Transcept said the drug is a lower dose formulation of zolpidem tartrate (Ambien), which was approved by Food and Drug Administration in 1992 for the short-term treatment of insomnia characterized by difficulties with sleep initiation.

Transcept said the recommended and maximum dose of Intermezzo is 1.75 milligrams for women and 3.5 mg for men, taken once per night.

"With this lower dose, there is less risk of a person having too much drug in the body upon waking, which can cause dangerous drowsiness and impair driving," Robert Temple, M.D., deputy center director for clinical science in the FDA's Center for Drug Evaluation and Research, said.

The drug, which has to be placed under the tongue, to let it disintegrate completely before swallowing, is designed to be taken only in the middle of the night.

Patients must have at least four hours of bedtime remaining after its intake, Transcept said in a statement.

The company had been trying to get an FDA clearance on Intermezzo for the past two years, and has faced rejection twice on lingering safety issues.

The drug targets a type of insomnia that affects millions of adults in the United States, Transcept said.

Transcept, which specializes in developing therapies in the field of neuroscience, said its marketing partner Purdue Pharmaceuticals had until Dec. 8 to notify if it would proceed with the commercialization.