Question: What causes Alzheimer's disease? Since it does sometimes run in families, do you think it might be contagious?

Answer: There doesn't appear to be one specific cause for Alzheimer's disease, the most common cause of dementia. It occurs due to a complex mix of genetic, lifestyle, and environmental factors.

Two factors are well known: advancing age and family history. If you are lucky enough to make it to 85 years of age, there's a 50-50 chance of getting Alzheimer's disease. And folks who have a parent or sibling with Alzheimer's disease are two to three times more likely to develop the disease than those with no family history.

There are also two genes (so far) that have been identified that increase the likelihood of developing Alzheimer's: Apo E4 and a very rare "deterministic" gene associated with early Alzheimer's disease. One good piece of news is that if you're of Indian ancestry, you may have a gene variant called Apo E2 that somehow protects against Alzheimer's.

New research conducted at University of Texas Medical School in Houston suggests that in some cases, Alzheimer's may result from an infection by viral particles called "prions." Prions have also been linked to mad cow disease. In the research, mice injected with human brain tissue from Alzheimer's patients developed the disease.

These preliminary findings do not suggest that casual contact could transmit Alzheimer's disease - just like you can't get HIV from such contact.

Q: I think my teenage son stays up way too late, and it's been a struggle to get him to go to bed at a decent hour. He insists that he gets enough sleep. Don't you agree that Ben Franklin's advice of "early to bed, early to rise" would be better for him?

A: His sleep behavior is pretty typical for a teenager. There's a biological explanation for it, too: An adolescent's internal 24-hour clock is different from a mature adult's in that the preferred times for falling asleep and waking are delayed. Your son's internal clock prevents him from feeling sleepy until later.

School start times are much earlier than the typical adolescent's naturally preferred wake time, so many high school students wind up nodding off in class. Teens try to catch up on sleep on the weekends, but sleeping late can make the situation worse by shifting the internal clock further out of sync with the school schedule.

A recent Australian study of children ages 9 to 16 found that those who went to bed late and slept late were 1.5 times more likely to be overweight or obese than children who went to bed early and awoke early.

Although you are fighting both your son's internal clock and a teenager's will, getting him to shift over several weeks to Ben Franklin's early to bed (9-10 p.m.) and early to rise (6 a.m.) would have benefits. Studies suggest these include feeling less sleepy upon waking; better mental acuity; improved daytime productivity; more positive mood; improved stamina; and a healthier immune system to protect against colds and viruses.

Mitchell Hecht i specializes in internal medicine. Send questions to him at: "Ask Dr. H," P.O. Box 767787, Atlanta, GA 30076. Due to the large volume of mail received, personal replies are not possible.