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.
• Females who are not of childbearing potential must be postmenopausal for 1 year or have history of hysterectomy and/or oophorectomy.
• Primary insomnia as defined by the DSM-IV-TRTM for at least 3 months and as defined by subjective sleep latency (sSL) greater than or equal to 30 minutes, subjective total sleep time (sTST) less than or equal to 6.5 hours per night, and daytime complaint(s) associated with disturbed sleep.
• Mean latency of greater than or equal to 20 minutes on 2 consecutive screening nights with neither night less than 15 minutes. Also, a mean of 60 minutes of wake time during the 480 minutes in bed across two nights with no night less than 45 minutes screening nights with neither night less than 15 minutes. Also, a mean of 60 minutes of wake time during the 480 minutes in bed across two nights with no night less than 45 minutes.
• Habitual bedtime is between 8:30 p.m. and 12:00 a.m.
• 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 night single-blind study medication, whichever is longer.
• Sleep schedule changes required by employment (e.g. shift worker) within three months prior to the first night 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 night of single blind study medication.
• Ever had a history of seizures, sleep apnea, chronic obstructive pulmonary disease, 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, as defined in Diagnostic and Statistical Manual of Mental Disorders, 4th Edition Revised and/or regularly consumes 4 or more alcoholic drinks per day.
• 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 night of single-blind study medication.
• Uses tobacco products during nightly awakenings.
• Used melatonin, or other drugs or supplements known to affect sleep/wake function within 1 week (or 5 half lives of the drug, whichever is longer) prior to the first day of single-blind study medication.
• Used any central nervous system medication within 1 week (or 5 half lives of the drug, whichever is longer) prior to the first day of single-blind study medication. These medications must not have been used to treat psychiatric disorders.
• 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.
• Positive urine drug screen including alcohol at screening or a positive breathalyzer test at each check-in.
• Apnea hypopnea index (per hour of sleep) greater than 10 as seen on polysomnogram, on the first night of the polysomnogram screening.
• Periodic leg movement with arousal index (per hour of sleep) greater than 10 as seen on polysomnogram, on the first night of polysomnogram screening.

• Any additional condition(s) 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, including:

  • Anxiolytics
  • Hypnotics
  • Antidepressants
  • Anticonvulsants
  • Sedating H1 antihistamines
  • Systemic steroids
  • Respiratory stimulants (eg, theophylline)
  • 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
  • All beta blockers
  • Melatonin
  • St. John's Wort
  • Kavakava
  • Gingko biloba