Which gland secretes melatonin and helps control sleep cycles?

The pineal gland is responsible for secreting melatonin, which plays a crucial role in regulating sleep-wake cycles and circadian rhythms. It responds to light stimuli, increasing melatonin production in the absence of light, thereby promoting sleepiness and helping to establish a pattern of sleep. The pineal gland is situated in the brain and is often referred to as the "third eye" due to its sensitivity to light.

The other glands mentioned, while vital for various functions in the body, do not produce melatonin. The thyroid gland primarily regulates metabolism through hormones like thyroxine. The adrenal gland produces hormones such as cortisol and adrenaline, which are more involved in stress responses and metabolic processes. The pituitary gland, located at the base of the brain, is often called the "master gland" because it regulates other endocrine glands but does not secrete melatonin. Thus, the pineal gland’s specific role in melatonin production makes it the correct answer in this context.