Melatonin is a molecule that is produced in the pineal gland and secreted into the blood. melatonin then travels to different parts of the brain. it causes drowsiness and decreases body temperature to assist in the regulation of the sleep/wake cycle. what kind of control is used to regulate this molecule?

By altering the light factor

Explaination:

Blood levels of the pineal hormone melatonin are high at night and low during the day. Its secretion is controlled by a rhythm-generating system located in the suprachiasmatic nucleus of the hypothalamus, which is in turn directed by light. So by designing a control in which the intensity or duration of light is altered to regulate this cycle.

It is produced in the presence of sympathetic noradrenergic stimulation synchronized to the ambient lighting cycle.  

Explanation:

Melatonin (N-acetyl-5-methoxy tryptamine) is an indolaminergic hormone (PM = 232.3) synthesized from tryptophan and derived from serotonin after two enzyme transformations that acetylate and replace the hydroxyl group with methoxy. Melatonin is produced by the pineal gland in the presence of sympathetic noradrenergic stimulation, through postganglionic innervation originated in the upper cervical ganglion.  Unlike the hormones dependent on the hypothalamus-pituitary axis, melatonin production is not subject to feedback mechanisms and, therefore, their plasma concentration does not regulate their production.

On the other hand, an essential functional feature of this system is that it is strictly controlled by the circadian rhythm system which means, that daily melatonin production obeys precisely a circadian rhythmic production synchronized to the ambient lighting cycle,  characteristic of day and night.  Moreover, this daily rhythmic production is such, that in any considered species, the peak of production occurs at night.

B. after several generations, this trait is likely to be much more common because individuals with the trait will have greater chance of passing it onto their offspring