Inclusion Criteria

• Females of childbearing potential who are sexually active must agree to use adequate contraception, and can neither be pregnant nor lactating from Screening throughout the duration of the study.
• Has had primary insomnia as defined by the Diagnostic and Statistical Manual of Mental Disorders, Text Revision and a history of daytime complaint(s) associated with disturbed sleep.
• Subjective sleep latency greater than or equal to 45 minutes and a subjective total sleep time less than or equal to 6.5 hours per night for at least 3 nights during the week of the lead-in period, based on subject diary.
• Habitual bedtime is between 8:30 PM and 12:00 AM.
• Body mass index between 18 and 34, inclusive.

Exclusion Criteria

• Known hypersensitivity to TAK-375 or related compounds, including melatonin.
• Previously participated in a study involving TAK-375.
• Participated in any other investigational study, and/or taken any investigational drug within 30 days or five half-lives prior to the first day of single-blind study medication, whichever is longer.
• Sleep schedule changes required by employment (eg, shift worker) within three months prior to the first day of single-blind study medication, or has flown across greater than three time zones within seven days prior to screening.
• Participated in a weight loss program or has substantially altered their exercise routine within 30 days prior to the first day of single-blind study medication.
• Has ever had a history of seizures, sleep apnea, chronic obstructive pulmonary disease, restless leg syndrome, schizophrenia, bipolar disorder, mental retardation, or cognitive disorder.
• History of psychiatric disorder (including anxiety or depression) within the past 12 months.
• History of drug addiction or drug abuse within the past 12 months.
• History of alcohol abuse within the past 12 months.
• Current significant neurological, hepatic, renal, endocrine, cardiovascular, gastrointestinal, pulmonary, hematologic or metabolic disease unless currently controlled and stable with protocol-allowed medication 30 days prior to the first day of single-blind study medication.
• Uses tobacco products during nightly awakenings.
• Any clinically important abnormal finding as determined by a medical history, physical examination, electrocardiogram, or clinical laboratory tests, as determined by the investigator.
• Positive hepatitis panel.

• Any additional condition that in the Investigator's opinion would:

  • Affect sleep-wake function
  • Prohibit the subject from completing the study
  • Not be in the best interest of the subject.

• Is required to take or continues taking any disallowed medication, prescription medication, herbal treatment or over-the counter medication that may interfere with evaluation of the study medication, including:

  • melatonin or other drugs or supplements known to affect sleep/wake function.
  • anxiolytics
  • hypnotics
  • antidepressants
  • anticonvulsants
  • sedating H1 antihistamines
  • systemic steroids
  • respiratory stimulants and decongestants
  • over-the-counter and prescription stimulants
  • over-the-counter and prescription diet aids
  • central nervous system active drugs (including herbal preparations with central nervous system effects)
  • narcotic analgesics
  • beta blockers
  • melatonin
  • St. John's Wort
  • kava-kava
  • gingko biloba