The primary purpose of the study is to evaluate the safety and tolerability following multiple ascending doses of E2086 in healthy adult participants.

Inclusion Criteria:

1. Non-smoking, male or female, aged greater than or equal to (>=) 18 years to less than or equal to (<=) 55 years (>=20 years to <=55 years for Japanese participants) at the time of informed consent. To be considered non-smokers, participants must have discontinued smoking for at least 4 weeks before dosing
2. Japanese participants must have been born in Japan of Japanese parents and Japanese grandparents, must have lived no more than 5 years outside of Japan, and must not have changed their lifestyle or habits, including diet, while living outside of Japan
3. Body mass index (BMI) >=18 to less than (<) 30 kilogram per square meter (kg/m^2) at screening only for Part A
4. Reports regular bedtime, defined as the time the participant attempts to sleep, between 22:00 and midnight
5. Reports regular waketime, defined as the time the participant gets out of bed for the day, between 05:00 and 10:00

Exclusion Criteria:

1. Females who are breastfeeding or pregnant at screening or baseline (as documented by a positive beta-human chorionic gonadotropin [ß-hCG] (or human chorionic gonadotropin [hCG]) test with a minimum sensitivity of 25 international units per liter (IU/L) or equivalent units of ß-hCG [or hCG]). A separate baseline assessment is required if a negative screening pregnancy test was obtained more than 72 hours before the dose of study drug
2. All females who are of childbearing potential: All females will be considered to be of childbearing potential unless they are postmenopausal (amenorrheic for at least 12 consecutive months, in the appropriate age group, and without other known or suspected cause) or have been sterilized surgically (that is, bilateral tubal ligation, total hysterectomy, or bilateral oophorectomy, all with surgery at least 1 month before dosing)
3. Males who have not had a successful vasectomy (confirmed azoospermia) or their female partners do not meet the criteria above (that is, not of childbearing potential) or practicing highly effective contraception throughout the study period or for 28 days after study drug discontinuation. No sperm donation is allowed during the study period and for 92 days after study drug discontinuation
4. Clinically significant illness that requires medical treatment within 8 weeks or a clinically significant infection that requires medical treatment within 4 weeks of dosing
5. Evidence of disease that may influence the outcome of the study within 4 weeks before dosing; example, psychiatric disorders and disorders of the gastrointestinal tract, liver, kidney, respiratory system, endocrine system, hematological system, neurological system, or cardiovascular system
6. Any history of surgery that may affect pharmacokinetic (PK) profiles of E2086, example, hepatectomy, nephrotomy, digestive organ resection or participants who have a congenital abnormality in metabolism
7. Any clinically abnormal symptom or organ impairment found by medical history at screening, and physical examinations, vital signs, ECG finding, or laboratory test results that require medical treatment at screening or baseline
8. Evidence of clinically significant disease (example, cardiac, respiratory, gastrointestinal, renal disease, sleep disorders) that in the opinion of the investigators could affect the participant's safety or interfere with the study assessments
9. A prolonged QT/QTc interval (QTcF greater than [>] 450 milliseconds [ms]) demonstrated on ECG at screening or baseline (based on average of triplicate ECGs). A history of risk factors for torsade de pointes (example, heart failure, hypokalemia, family history of long QT Syndrome) or the use of concomitant medications that prolonged the QT/QTc interval
10. Left bundle branch block at screening or baseline
11. Persistent systolic blood pressure (BP) >130 or <100 millimeters of mercury (mmHg) or diastolic BP >85 or <50 mmHg at screening or baseline (based on BP measured on at least 3 occasions over 2 weeks)
12. Persistent heart rate (HR) <50 bpm or >100 bpm at screening or baseline (based on HR measured on at least 3 occasions over 2 weeks)
13. History of myocardial infarction, ischemic heart disease, or cardiac failure
14. History of clinically significant arrhythmia or uncontrolled arrhythmia
15. Any lifetime history of suicidal ideation or any lifetime history of suicidal behavior as indicated by the C-SSRS
16. Any lifetime history of psychiatric disease (including but not limited to depression or other mood disorders, bipolar disorder, psychotic disorders, including schizophrenia, panic attacks, anxiety disorders)
17. Any current psychiatric symptoms as indicated by a standard screening tool (Diagnostic and Statistical Manual of Mental Disorders Self-Rated Level 1 Cross-Cutting Symptom Measure - Adult)
18. Participants with 1 or more first degree (blood) relatives who have lifetime diagnosis of bipolar type I disorder or a psychotic disorder
19. Receipt of blood products within 4 weeks, or donation of blood within 8 weeks, or donation of plasma within 1 week of dosing
20. History of formally diagnosed moderate to severe obstructive sleep apnea, current use of continuous positive airway pressure, or symptomatic restless legs syndrome

Eisai's data sharing commitment and further information on how to request data can be found on our website http://eisaiclinicaltrials.com/.

Part A is double-blind and Part B is open-label.