Pharmacokinetic and Safety of Ramelteon Between Adolescents With Insomnia and Healthy Adults

Inclusion Criteria: Inclusion criteria for adolescent and pediatric participants only: * Is male or female between 12 and 17 years of age (less than 18 years of age on Day 1) with complaints of insomnia characterized by difficulty with sleep initiation OR a male or female between 6 to 11 years of age (less than 12 years of age on Day 1) with complaints of insomnia characterized by difficulty with sleep initiation associated with ADHD. * Has a body mass index within the 5th to 95th percentile of the appropriate body mass index designated charts based on stature-for-age and weight-for-age and by gender. * In the age group of 12 to 17 years, has a history of primary insomnia characterized by difficulty initiating sleep as defined by the Pharmacologic Management of Insomnia in Children and Adolescents: Consensus Statement OR in the age group of 6 to 11 years, has a history of insomnia characterized by difficulty with sleep initiation (as defined by the Pharmacologic Management of Insomnia in Children and Adolescents: Consensus Statement associated with ADHD). * There is agreement in the participant's parent or caregiver's opinion with the following: * The complaint involves significant difficulty in initiating sleep * The sleep disturbance does not occur exclusively during the course of narcolepsy, breathing-related sleep disorder, circadian rhythm sleep disorder, or parasomnia. * The disturbance does not occur exclusively during the course of another mental disorder (eg, Major Depressive Disorder, Generalized Anxiety Disorder, and Delirium). * The disturbance is not due to the direct physiological effects of a substance (eg, a drug of abuse, a medication) or a general medical condition. * Based on sleep history, reports a subjective sleep latency greater than or equal to 45 minutes for at least 1 month. * If taking concomitant medications, he/she has been on a stable dose or regimen of his/her medication for at least 30 days prior to Screening. Inclusion criteria for gender- and race-matched adult participants only: * Weighs at least 50 kg (110 pounds) and has a Screening body mass index between 18 and 30 kg/m\^2, inclusive. Inclusion criteria for all participants: * A female of childbearing potential (defined as females aged ≥12 years old and younger girls who, at the discretion of the investigator, are deemed to be of reproductive potential) and males who are sexually active agree to routinely use adequate contraception from Screening throughout the duration of the study and through 30 days following the last dose of study medication. * Must have a negative urine test result for selected substances of abuse (including alcohol) at Screening and Day 1. * Has clinical laboratory results (including clinical chemistry, hematology, and complete urinalysis \[fasted\] within the reference range for the testing laboratory unless the results are deemed not clinically meaningful by the investigator or sponsor. * Has a negative test result for hepatitis B surface antigen and hepatitis C virus antibody, and no known history of human immunodeficiency virus. Exclusion Criteria: * Is participating in another investigational study or has taken an investigational drug within 30 days (or 5 half-lives, whichever period is longer) prior to study Screening. * Has received ramelteon within 30 days of Screening. * Is a study site employee, or is an immediate family member (ie, spouse, parent, child, sibling) of a study site employee, involved in conduct of this study. * Has abnormal hematological parameters of hemoglobin and/or hematocrit (if these exceed +/- 2 points of the normal range for the age and sex appropriate values), or erythrocytes at Screening. * Has a known hypersensitivity to ramelteon or related compounds including melatonin. * Has a history of drug abuse (defined as any illicit drug use) or a history of alcohol abuse (defined as consumption of more than 4 alcoholic drinks per day) within 1 year prior to study Day 1. * Has had an acute, clinically significant illness within 30 days prior to Screening. * Has autistic spectrum disorders or other pervasive developmental disorder. * Has a history or clinical manifestations of significant metabolic (including diabetes mellitus, hypercholesterolemia, or dyslipidemia), hematologic, pulmonary, cardiovascular, gastrointestinal, neurologic, hepatic, renal, urologic, immunologic, musculoskeletal, or psychiatric disorder unless currently controlled and stable with protocol-allowed medication for at least 30 days prior to Screening (except for ADHD in the age group of 6 to 11 years). * Has sleep schedule changes required by employment, school and/or extra curricular activity (eg, shift worker) within 3 months prior to Screening, or has flown across greater than 3 time zones within 7 days prior to Screening. * Has a history or clinical manifestations of depression, seizures, sleep apnea, restless leg syndrome, or periodic leg movements during sleep. * Has a history of abdominal surgery (except laparoscopic cholecystectomy or uncomplicated appendectomy) or thoracic or nonperipheral vascular surgery within 6 months prior to study Day 1. * Has a history of cancer, other than basal cell carcinoma or Stage 1 squamous cell carcinoma of the skin that has not been in remission for at least 5 years prior to study Day 1. * Has used any tobacco (ie, nicotine) products (including but not limited to cigarettes, pipe, cigar, chewing tobacco, nicotine patch, or nicotine gum) within 6 weeks prior to Screening, or is unwilling to abstain from these products for the duration of the study. * Has poor peripheral venous access. * Has any clinically important abnormal finding as determined by a medical history, physical examination, electrocardiography (ECG), or clinical laboratory tests, as determined by the investigator. Participants with clinically significant abnormalities being considered for the study must be approved by both Takeda medical monitor or designee and the Principal Investigator. * Has any additional condition(s) that in the Investigator's opinion would: a) affect sleep/wake function, b) prohibit from completing the study, or c) not be in the best interest of to participate in the study.