The smooth, thin membrane that lines the chambers of the heart and forms the surface of the heart valves is the:

Hormones that the posterior pituitary secretes are synthesized in the hypothalamus.

They are made by neurons in the hypothalamus and stored and released by the posterior lobe of the pituitary gland, a small organ at the base of the brain.
They regulate water balance, blood pressure, and uterine contraction in the body.
Choice A is wrong because the anterior pituitary synthesizes its own hormones, such as growth hormone, thyroid-stimulating hormone, and prolactin.
Choice C is wrong because the thalamus is not involved in hormone synthesis or secretion. It is a part of the brain that relays sensory and motor signals to the cerebral cortex.
Choice D is wrong because the posterior pituitary does not synthesize hormones. It only stores and releases hormones made by the hypothalamus.

Circadian rhythm is the term for the biological clock that regulates the sleep-wake cycle and other physiological processes in the body.
Melatonin is a hormone that is produced by the pineal gland in the brain in response to darkness.
It helps with the timing of the circadian rhythm and promotes sleep.
Choice A is wrong because metabolic rate is not affected by light exposure, and thyroxine is a hormone that regulates metabolism, not sleep.
Choice B is wrong because the limbic system is a part of the brain that is involved in emotions, memory, and motivation, not sleep.
Thymosin is a hormone that stimulates the development of T cells, which are part of the immune system.
Choice C is wrong because the circulatory system is the system of blood vessels and organs that transport blood throughout the body, not a part of the brain that controls sleep.
Dopamine is a neurotransmitter that is involved in reward, motivation, and movement, not sleep.