• Average subjective sleep latency, per subject diary, from Nights 1 through 7 of double-blind treatment. [ Time Frame: 35 Days ] [ Designated as safety issue: No ]
  

• Objective and subjective measures of efficacy [ Time Frame: 35 Days ] [ Designated as safety issue: No ]
  
• Assessment of effects of withdrawal and rebound insomnia. [ Time Frame: 35 Days ] [ Designated as safety issue: No ]
  
• Safety variables including adverse events, laboratory tests, vital signs, ECG results, and physical exam findings. [ Time Frame: 35 Days ] [ Designated as safety issue: Yes ]
  

Subjects who met the study criteria entered a 35-night, Double-Blind Treatment Period. During the Double-Blind Treatment Period, subjects were randomized to receive TAK-375 4 mg, TAK-375 8 mg, or placebo. This period was followed by a 7-night, Single-Blind Placebo Run-out Period to evaluate possible rebound insomnia and withdrawal effects. Throughout the course of the study, subjects were asked to maintain daily diaries and return to the clinic weekly for Clinical Global Impressions (CGI), and review of sleep diaries, concomitant medications, and adverse events.

Inclusion Criteria:

• The subject was a man or a postmenopausal woman.
• The subject had primary insomnia as defined by the DSM-IV-TRTM for at least 3 months and a history of daytime complaint(s) associated with disturbed sleep.
• 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 prior to the first day 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 the first day of single-blind study medication, or had flown across greater than 3 time zones within 7 days prior to Initial Screening.
• The subject had participated in a weight loss program or had substantially altered his/her exercise routine within 30 days prior to the first day of single-blind study medication.
• The subject had a history of seizures, sleep apnea, chronic obstructive pulmonary disease (COPD), restless leg syndrome, 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 used tobacco products during nighttime awakenings.
• The subject had used melatonin, or other drugs or supplements known to affect sleep/wake function within 1 week (or 5 half lives of the drug, whichever was longer) prior to the first day of single-blind study medication.
• The subject had used any CNS medication within 1 week (or 5 half lives of the drug, whichever was longer) prior to the first day of single-blind study medication. These medications must not have been used to treat psychiatric disorders.