Doxepin isomers and isomeric mixtures and methods of using the same to treat sleep disorders

A method for treating insomnia, the method comprising administering an oral formulation comprising doxepin, or a pharmaceutically-acceptable salt thereof, to a patient having a sleep disorder in which, for a given 8 hour period of desired sleep, the patient has difficulty staying asleep during the final 60 minutes of said period, wherein the oral formulation comprises a dosage of doxepin ranging from about 1 to about 7 milligrams, wherein the doxepin or salt is a geometric isomer mixture containing about 18.2% to about 100.0% of the cis-(Z) isomer or is 100% cis (Z) isomer.