The pineal body is believed to produce which substance?

The pineal body, or pineal gland, is primarily known for producing melatonin, a hormone that plays a crucial role in regulating sleep-wake cycles and circadian rhythms. Melatonin is synthesized from the neurotransmitter serotonin during the night, and its levels in the blood can increase significantly in response to darkness, signaling the body to prepare for sleep.

Melatonin's primary function is to help regulate sleep patterns and synchronize the biological clock, making it essential for maintaining a healthy sleep schedule. This hormonal effect is particularly important in managing seasonal changes in light exposure, which can affect reproductive hormones and seasonal breeding in some animals.

The other choices do not pertain to the function of the pineal gland. Insulin is produced by the pancreas and regulates blood sugar levels, cretinism is a condition associated with hypothyroidism in childhood, and metabolism refers to all the biochemical processes in the body for energy production and utilization, which are not directly linked to the pineal body.