If you have been parked in a chair for hours, this study has a pretty simple message: get up and move, and try to get about seven hours of sleep tonight.

Regular exercise and about seven hours of sleep a night could protect brain health in the long term, while long bouts of sedentary behavior may increase dementia risk, according to a study published Wednesday in the journal PLOS One.

The study found simple lifestyle changes could reduce a person’s risk of late-onset dementia by as much as 25 percent.

About 1 in 9 people in the United States will develop Alzheimer’s disease, according to the Alzheimer’s Association, meaning a person’s overall risk is about 11 percent. With the suggested changes in lifestyle, the average person’s risk decreases to about 8 percent.

The reduction is “fairly comparable to the effect sizes sometimes seen with medications for chronic diseases,” said Akinkunle Oye-Somefun, a researcher at York University in Toronto, who led the study.

Breaking up longer periods of sitting had the greatest effect, the study found.

“You don’t need to be perfect, but if you have a lot of sedentary time in your day, getting a bit of exercise, even if that’s just going for a walk, will have a long-term benefit for the brain,” said Phillip Hwang, an assistant professor of epidemiology at the Boston University School of Public Health, who was not involved in the study.

Researchers used data from 69 studies that included nearly 3 million people over eight decades. The average age of the people in the studies was 67 years old and they all lived in high-income countries, including the United States.

The studies followed people for at least a year, and sometimes as long as 11 years. They measured how sleeping, sitting and activity habits starting at age 35 affected dementia risk later in life.

Seventeen of the studies focused on sleep. The study found the sweet spot for brain health was getting between seven and eight hours of sleep each night.

Sleeping less than seven hours increased dementia risk by 18 percent. Getting more than eight hours of sleep a night increased dementia risk by 28 percent.

The rest of the studies looked at dementia risk and either physical activity or sedentary behavior, such as sitting at work for hours at a time. Together, they showed that sitting for more than eight hours a day increased dementia risk by almost 30 percent, while regularly being active, even just going for a walk every day, decreased dementia risk by an average of 25 percent.

Moderate physical activity “offsets dementia risks even when other risk factors are present,” Oye-Somefun said.

He also said long periods of sitting can increase a person’s propensity to cardiovascular disease, diabetes and obesity, all of which raise dementia risk.

However, “many people assume that being physically active cancels out the harm of sitting for long periods. It doesn’t,” Oye-Somefun said. “We shouldn’t do one of these things alone, we should do them all.”

Amal Wanigatunga, an assistant professor of epidemiology at Johns Hopkins Bloomberg School of Public Health who was not involved in the research, said exercising before work and then sitting for eight hours or more can erase some of the brain benefits of exercise.

He said exercise boosts blood flow and oxygen to the brain, and breaking up long periods of sitting by standing or walking can improve brain blood flow.

“The brain has these very small capillaries, so if there is reduced blood flow, especially to these very small pathways, that can be the bedrock for brain atrophy,” he said.

Wanigatunga said muscle contractions, which occur when the body is moving, release a protein called brain-derived neurotrophic factor, or BDNF. He said that molecule boosts cognitive function, generates new neurons and improves connections between neurons, especially in the hippocampus, the part of the brain that converts short-term memory to long-term memory.

Hwang said physical activity has also been shown to reduce a peptide called beta-amyloid, which causes plaque to build up in the brain and may play a role in Alzheimer’s disease and dementia. Oye-Somefun said exercise also reduces chronic inflammation in the brain, which hinders its ability to repair itself.

“Whatever the underlying cause for dementia might be, it does seem that physical activity can have a beneficial impact on the brain,” Hwang said.

The study also cited research suggesting that staying mentally active while sitting, such as solving a puzzle or knitting, could help keep brains engaged even when someone is sedentary. But Oye-Somefun said mental stimulation provides different protection for the brain than physical activity, making both important.

Finally, Wanigatunga said consistent physical activity matters more than intense exercise.

“Once you have it as a habit, you can refine it,” Wanigatunga said.