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You might want to train your body to develop a steady routine sleep schedule. Four new studies claim that the amount of sleep and the quality could cause mental deterioration and Alzheimer's disease.

The new findings are not yet peer-reviewed and published in a medical journal, and so the results are considered preliminary.

According to one study, a person's brain can age up two years if they get too much or two little sleep. Another study suggests that people with sleep apnea were more than twice as likely to develop mild thinking problems or dementia compared to problem-free sleepers. On the other hand, one study claims that excessive daytime sleepiness may predict diminished memory and thinking skills.

Heather Snyder, senior associate director of medical and scientific relations for the Alzheimer's Association in Chicago, was not involved in the studies and she said researchers may need to dig deeper.

"Whether sleep changes, such as sleep apnea or disturbances, are signs of a decline to come or the cause of decline is something we don't know," Snyder said. "But these four studies... shed further light that this is an area we need to look into more."

One of the studies looked and 15,000 women and claimed that those who slept five hours a day or less, or nine hours a day or more, had lower average mental functioning than participants who slept seven hours per day.

Researchers followed the women for 14 years and said too much or too little sleep was cognitively equivalent to aging by two years.

Study Author Elizabeth Devore said these new research will allow researchers to figure out the physical, mental and behavioral changes that follow a 24-hour cycle.

"We went in with the hypothesis that extreme changes in sleep duration might be worse for cognitive function because they disrupt the circadian rhythm, so these results line up nicely," Devore said. "I think this gives us data to think about sleep- and circadian-based interventions being a route to address cognitive function."

University of California scientists found that participants with sleep-disordered breathing or sleep apnea had more than twice the odds of developing mild cognitive impairment or dementia over five years than those without those conditions. Those with greater nighttime wakefulness were more likely to score worse on tests of verbal fluency and global cognition. They studied over 1,300 women over 75 years old.

French researchers observed almost 5,000 "mentally healthy French people" who were over age 65. They were evaluated four times in eight years and researchers concluded that that excessive daytime sleepiness -- which was reported by 18 percent of participants -- increased the risk of mental decline. Difficulty in staying asleep did not.

Scientists from Washington University School of Medicine in St. Louis obtained samples of blood and cerebrospinal fluid from three groups of volunteers -- those with dementia, a healthy age-matched set and a younger set -- over 36 hours and found that daily sleep patterns were linked to levels of amyloid proteins. These proteins are recognized as an indicator of Alzheimer's disease.

Snyder and Devore believe that these findings could help prevent mental deterioration.

"We may be able to help those individuals," Snyder said. "If you're having problems with sleep, you may want to follow up with your health care provider."

Researchers siad lthough the studies report an association between sleep disturbances and mental decline, they do not show a cause-and-effect relationship.