The purpose of this study is to assess the safety and tolerability of TAK-994 following single and multiple oral doses in healthy non-Japanese and Japanese adult participants and healthy elderly participants.

The drug being tested in this study is called TAK-994. TAK-994 is being tested to find a safe and well-tolerated dose in healthy participants (non-Japanese and Japanese) and healthy elderly participants. The study will enroll up to approximately 160 healthy participants. The study consists of 6 parts and up to 20 cohorts as mentioned below.

• TAK-994: Part A: Single-rising dose (SRD) design to assess the safety, tolerability, and PK of TAK-994 and effect of food on the PK of the TAK-994
• TAK-994: Part B: MRD design to assess the safety, tolerability, PK and PD of TAK-994
• TAK-994: Part C: MRD design to assess the safety, tolerability, PK and PD of TAK-994 and also central nervous system penetration relative to plasma concentrations of TAK-994
• TAK-994: Part D: MRD design to assess the safety, tolerability, PK and PD of TAK-994 for HE participants
• TAK-994: Part E: SRD and MRD design to assess the safety, tolerability, PK and PD of TAK-994 for Japanese origin participants
• TAK-994: Part F (Optional Cohort): Nonrandomized design to assess orexin (OX) levels in the cerebrospinal fluid (CSF) of untreated healthy adult participants.

Participants in each cohort will be randomized to receive treatment with TAK-994 or matching placebo which will remain undisclosed to the participant and study doctor during the study (unless there is an urgent medical need). Participants in optional Part F will receive no study drug.

This multi-center trial will be conducted in the United States. The overall duration of the study is approximately 10 months. Participants will be followed up for 7 days after the last dose of study drug for a follow-up assessment.

Inclusion Criteria:

1. Be normotensive, with no history of hypertension or use of antihypertensive medication. Blood pressure (BP) must be less than (<) 140 millimeter of mercury (mmHg) (systolic) and <90 mmHg (diastolic).

   Healthy Adult and Elderly Participants (Parts A through D and Part F)

2. Must have a body mass index (BMI) greater than or equal to (>=) 18.0 and less than or equal to (<=) 30.0 kilogram per square meter (kg/m^2) at the screening visit (non-Japanese only).

   Healthy Adult Participants (Parts A, B, C, and F)

3. Must be aged 18 to 55 years, inclusive, at the screening visit.

4. Must have a body weight >=50 kilogram (kg) at the screening visit.

   HE Participants (Part D)

5. Must be aged >=65 years, inclusive, at the time of informed consent.

6. Must have a body weight >=40 kg at the screening visit.

   Healthy Japanese Adult Participants (Part E)

7. Must be aged 18 to 55 years, inclusive, at the screening visit.

8. Must have a BMI >=18.0 and <=26.0 kg/m^2 at the screening visit.

9. Must have been born in Japan to a Japanese mother and father and have maternal and paternal Japanese grandparents.

10. Must have not been away from Japan for more than 10 years at the screening visit.

11. In the opinion of the investigator, must have a lifestyle that has not changed significantly since relocation from Japan.

Exclusion Criteria:

1. Has a known hypersensitivity to any component of the formulation of TAK-994 or related compounds.

2. Has a risk of suicide according to endorsement of Item 4 or 5 of the screening/baseline visit Columbia Suicide Severity Rating Scale (C-SSRS) or has made a suicide attempt in the previous 6 months.

3. Has a lifetime history of major psychiatric disorder, such as bipolar disorder or schizophrenia. Participant who have history of major depressive disorder (MDD) may be included, but participants who have current active MDD or who have had active MDD in the past 6 months are excluded.

4. Has a clinically significant history of head injury or head trauma.

5. Has a history of cerebral ischemia, transient ischemic attack, intracranial aneurysm, or arteriovenous malformation.

6. Screening electrocardiogram (ECG) reveals a QT interval with the Fridericia's correction method (QTcF) greater than (>) 450 millisecond (ms) (men) or >470 ms (women).

7. Has a resting heart rate outside of the range of 45 to 100 beats per minute, confirmed on repeat testing within a maximum of 30 minutes).

   Healthy Non-Japanese Adult Participants (Part C and Part F)

8. Has undergone CSF collection within 30 days before check-in (Day -2 [Part C] or Day -1 [Part F]).

9. Has a known hypersensitivity to anesthesia or its derivatives used during CSF collection or to any medication used to prepare the area of lumbar puncture.

Takeda makes patient-level, de-identified data sets and associated documents available after applicable marketing approvals and commercial availability have been received, an opportunity for the primary publication of the research has been allowed, and other criteria have been met as set forth in Takeda's Data Sharing Policy (see www.TakedaClinicalTrials.com/Approach for details). To obtain access, researchers must submit a legitimate academic research proposal for adjudication by an independent review panel, who will review the scientific merit of the research and the requestor's qualifications and conflict of interest that can result in potential bias. Once approved, qualified researchers who sign a data sharing agreement are provided access to these data in a secure research environment.