Doxepin trans 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 between about 1 and about 7 mg, wherein the doxepin or salt thereof is a geometric isomer mixture containing about 88.3% to about 100.0% of the trans-(E) isomer or is 100% trans-(E) isomer, and wherein the dosage is administered prior to the start of the sleep period.