Melatonin May Help Prevent Alzheimer’s Disease

This one you'll have to read and decide with your doctors help. I do take melatonin but it is mainly for helping me sleep.
http://www.life-enhancement.com/magazine/article/1738-melatonin-may-help-prevent-alzheimers-disease?
A few paragraphs;

A Melatonin “Laundry List”
Let’s outline some documented scientific facts and expert opinions regarding melatonin and Alzheimer’s disease (AD) and then see what they add up to. Most of the information that follows is from two recent review papers, one by scientists in China² and the other by a team from Malaysia, the United States, Argentina, and Germany.³

• In our forties or fifties, typically, our nightly melatonin output begins a steep decline from its youthful levels—a decline that correlates with physical deterioration of the pineal gland itself. By our late seventies, melatonin output has dropped to very low levels.
• Accounting for age, abnormally low levels of melatonin in the blood and cerebrospinal fluid are observed even in the early, preclinical stages of AD, suggesting that this hormone may be a useful early marker for the disease. As AD progresses, the levels decline further—dramatically—in correlation with the severity of the patients’ cognitive impairment.
  

  Sundown(ing)

• Melatonin output in AD can also be highly irregular, resulting in severe disruptions of normal circadian rhythms, including the sleep/wake cycle. This often manifests as sundowning, a decrease in cognitive functions and an increase in bizarre behaviors during the evening and nighttime hours. Melatonin has been successful in treating AD-related sleep disorders, the severity of which correlates with the severity of the patients’ cognitive impairment.
• Oxidative stress—the damage done to molecular and cellular entities by reactive oxygen species, including free radicals—is very strongly implicated in AD, as both a causative factor and an exacerbating factor. The brain is exceptionally vulnerable to oxidative stress, owing to its high rate of chemical energy generation (through cellular respiration, the primary source of free radicals) and its high content of polyunsaturated fatty acids, which are easily damaged by free radicals.
• Melatonin is a powerful antioxidant. Its actions occur not only through the scavenging of free radicals but also through a variety of other, less direct chemical mechanisms that reduce oxidative stress, including the upregulation of antioxidant enzymes and the downregulation of prooxidant enzymes. This makes melatonin more versatile and potent than conventional antioxidants. It also makes it extremely difficult for researchers to distinguish, where melatonin is concerned, between causal connections and mere correlations that may or may not mean anything.
• Melatonin’s antioxidative protection may also be related to its chronobiological role as a regulator of circadian rhythms, the disruption of which promotes abnormal levels of oxidative stress. This suggests the value of melatonin supplementation in the elderly, in whom the normal daily fluctuations in melatonin levels are greatly reduced, and especially in AD patients, in whom the daily fluctuations may, in addition, be highly irregular, as noted above.
• A significant advantage of melatonin over most other antioxidants is that its molecular structure allows it to cross the blood-brain barrier and to enter any component of the neurons (brain cells), including the mitochondria. These organelles, where the brain’s chemical energy is generated, are the chief source of free radicals—and they are its chief victims. Oxidative damage to the mitochondria is thought to be a centrally important factor in aging and dementia.
• Melatonin is neuroprotective, i.e., it helps protect brain neurons from damage or death caused by a variety of factors, notably oxidative stress. This protective effect is most pronounced in the mitochondria and is believed to be melatonin’s most important benefit.
• Melatonin is protective against neuritic plaques and neurofibrillary tangles, the two dominant neuropathological features of Alzheimer’s brains. Specifically, melatonin protects against the destructive effects of two dangerous proteins: amyloid-beta (the principal constituent of neuritic plaques) and hyperphosphorylated tau (of which neurofibrillary tangles are composed). Amyloid-beta attacks and destroys neurons, resulting in the substantial loss of brain matter seen in Alzheimer’s victims at autopsy. Hyperphosphorylated tau (rhymes with wow) is also destructive; it’s the result of a chemical modification of a benign (and vital) protein called simply tau.
• Melatonin is believed to promote neurogenesis (the formation of new neurons) by playing a role in the construction of the cytoskeleton, the neuron’s structural framework. One of the key constituents of the cytoskeleton is tau, which, when it becomes damaged through hyperphosphorylation, forms neurofibrillary tangles. Melatonin inhibits that process.
• Rats in which melatonin biosynthesis was inhibited (by means of the antipsychotic drug haloperidol) suffered impaired spatial memory. Administration of melatonin before and during the haloperidol treatment, however, significantly prevented memory loss. It also inhibited the hyperphosphorylation of tau, and it reduced oxidative stress. Those two benefits are probably related, because the former phenomenon is influenced by the latter.
  

  Typical fluctuations of core body temperature over a 24-hour period. This circadian rhythm is regulated by melatonin.

• Oxidative stress is also implicated in the formation of amyloid-beta from its precursor, amyloid precursor protein (APP), and amyloid-beta itself promotes further oxidative stress, playing a role in the promotion of tau hyperphosphorylation and the death of brain neurons. Melatonin regulates APP metabolism and prevents amyloid-beta pathology and neuronal death—but only, apparently, if given before the deposition of neuritic plaques (which are also called senile plaques) begins; after that, it’s too late.
• Amyloid-beta also has proinflammatory effects on brain tissue, mainly through its stimulation of molecules called cytokines and chemokines. Melatonin inhibits this activity and thus has an anti-inflammatory effect.
• Melatonin has some protective effect on the brain’s cholinergic nervous system, i.e., those parts of the neural circuitry that depend on acetylcholine as the neurotransmitter. A marked decline in cholinergic activity is primarily responsible for the memory loss and other cognitive deficits seen in Alzheimer’s patients.
• Melatonin is known to enhance immune-system function by promoting the production of T lymphocytes, a type of white blood cell that enhances the production of antibodies.
• Melatonin promotes regular, healthy sleep, which is considered to be neuroprotective through its fostering of improved neuronal metabolism. That’s especially important in preventing age-related neurodegenerative diseases or attenuating their progression.
• Melatonin has very low toxicity and can be taken by most people in large amounts (many milligrams daily) with no serious side effects. (As with many supplements, however, one must beware of possible interactions with prescription drugs; consult your doctor.) In the elderly, a common side effect of relatively large doses (3 mg or more) is reduced body temperature, the result of sudden alterations in melatonin’s circadian thermoregulatory function.