Inclusion Criteria:

• The subject was a man or a postmenopausal woman.
• The subject had primary insomnia as defined by the DSM-IV-TR™ for at least 3 months and by sSL greater than or equal to 30 minutes, sTST less than or equal to 6.5 hours per night, and daytime complaint(s) associated with disturbed sleep.
• The subject had a mean latency of greater than or equal to 20 minutes per PSG 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 2 nights with no night less than 45 minutes.
• The subject's habitual bedtime was between 8:30 PM and 12:00 AM.

Exclusion Criteria:

• The subject had a known hypersensitivity to TAK-375 or related compounds including melatonin.
• The subject had previously participated in a study involving TAK-375.
• The subject had participated in any other investigational study or taken any investigational drug within 30 days or 5 half-lives (T1/2s) prior to Day 1 of single-blind study medication, whichever was longer.
• The subject had sleep schedule changes required by employment (eg, shift worker) within 3 months prior to Day 1 of single-blind study medication, or had flown across greater than 3 time zones within 7 days prior to Screening.
• The subject had participated in a weight loss program or had substantially altered their exercise routine within 30 days prior to Day 1 of single-blind study medication.
• The subject had a history of seizures, sleep apnea, chronic obstructive pulmonary disease (COPD), schizophrenia, bipolar disorder, mental retardation, or cognitive disorder.
• The subject had a history of psychiatric disorder (including anxiety or depression) within the past 12 months.
• The subject had a history of drug addiction or drug abuse within the past 12 months.
• The subject had a history of alcohol abuse within the past 12 months, as defined in DSM-IV-TR™, or regularly consumed 4 or more alcoholic drinks per day.
• The subject had an acute clinically significant illness, as determined by the investigator, within 30 days prior to Day 1 of single-blind study medication.
• The subject had a current significant neurological (including cognitive and psychiatric disorders), hepatic, renal, endocrine, cardiovascular, gastrointestinal, pulmonary, hematologic, or metabolic disease, unless currently controlled and stable with protocol-allowed medication 30 days prior to Day 1 of single-blind study medication.
• The subject used tobacco products within 90 days prior to Day 1 of single-blind study medication.
• The subject had used melatonin, or other drugs or supplements known to affect sleep-wake function, or had consumed grapefruit or grapefruit juice within 5 days (or 5 T1/2s whichever is longer) prior to Day 1 of single-blind study medication.
• The subject had used any CNS medication within 3 weeks (or 5 T1/2s, of the drug, whichever was longer) prior to Day 1 of single-blind study medication. These medications must not have been used to treat psychiatric disorders.
• The subject intended to continue taking any disallowed medication or any prescription medication or OTC medication that is known to affect the sleep-wake function or otherwise interfere with evaluation of the study medication. The subject must have reported all prescription and OTC medications taken in the 3 weeks prior to Screening.
• The subject had any clinically important abnormal finding as determined by a medical history, physical examination, ECG, or clinical laboratory tests, as determined by the investigator. Subjects with clinically significant abnormal laboratory values being considered for the study had to be approved by both TPNA and the principal investigator.
• The subject had a positive hepatitis panel including hepatitis A antibody (only reactive IgM was exclusionary), hepatitis B surface antibody, hepatitis B core antibody (only reactive IgM was exclusionary), hepatitis C antibody, or hepatitis B surface antigen. Subjects who tested positive for hepatitis B surface antibody were not excluded if the result was due to vaccination.
• The subject had an apnea hypopnea index (per hour of sleep) greater than 15 as seen on PSG, on the first night of the PSG Screening.
• The subject had periodic leg movement (PLM) with arousal index (per hour of sleep) greater than 20 as seen on PSG, on the first night of PSG Screening.
• The subject had any additional condition(s) that in the investigator's opinion would: 1) affect sleep-wake function, 2) prohibit the subject from completing the study, or 3) not be in the best interest of the subject to participate in the study.