A. Stimulate immune function: The pineal gland does not have a direct role in immune function. While some research suggests melatonin may have minor immune-modulating effects, this is not its primary function. B. Regulate sleep-wake cycles: The pineal gland produces melatonin, a hormone that helps regulate circadian rhythms. Melatonin secretion increases in darkness and decreases in light, playing a key role in promoting sleep and maintaining sleep-wake cycles. C. Regulate metabolism: Metabolic regulation is primarily controlled by the thyroid gland, pancreas, and hypothalamus, not the pineal gland. The pineal’s influence on metabolism is indirect at best through circadian rhythm regulation. D. Control blood sugar levels: Blood sugar regulation is managed by the pancreas through insulin and glucagon secretion. The pineal gland does not directly affect glucose homeostasis.