Melatonin regulates the sleep cycle. If your endocrine system has become compromised or if your melatonin production itself is out of whack (staring at any source of light past your bedtime will cause this, IE computer monitor, as production is linked intimately with light perception) then you will probably not sleep normally and likely will develop some form of insomnia. (as if there is more than one kind.)
Not getting adequate sleep = greater chance of tumor growth caused by stress. Not to mention that your endocrine system produces many chemicals that simply inhibit tumor growth from the get go. Your thyroid is part of the endocrine system, for example. Having hyper/hypothyroidism as a result of a non optimal pineal gland efficiency would likely also be a factor in determining cellular health and possibly even the lifespan of an organism.
Bonus Info:
Melatonin is produced in the pineal gland, a small pine-cone shaped (hence, the name) organ near the base of your brain. The pineal gland is outside of the blood brain barrier, thus it is easily affected by free radical oxidation that is part of the "normal aging process." Heavy metals in particular tend to accumulate there, so it it important to keep your system well maintained by ingesting plenty of nutrient-rich foods and brightly colored vegetables and fruits that are high in antioxidants and other free radical scavenging molecules.
http://en.wikipedia.org/wiki/MelatoninThe dark side of light at night: physiological, epidemiological,
and ecological consequences
http://www.psy.ohio-state.edu/nelson/documents/JPinealRes2007.pdfEpidemiology of urinary melatonin in women and its relation to other hormones and night work.
http://www.ncbi.nlm.nih.gov/pubmed/15184249