Answer:
The correct answer will be- the Pineal gland
Step-by-step explanation:
The endocrine system is a system of glands which controls the body functions by secreting chemical messengers called the hormones.
The endocrine gland which is found in the middle part of the brain is called the pineal gland which is associated with the wake-sleep cycle of the body by producing a hormone called melatonin.
Thus, the pineal gland is the correct answer.