Sustained stress can cause Alzheimer's diseaese in advanced years. 

A research on mice has indicated there is an increased risk of dementia in case one remains constantly stressed. Sara Bengtsson, a PhD student from Umea University, in Sweden, tested her theory on mice and found that mice with higher levels of the hormone in their brains suffered impaired learning and memory, Daily Mail reported.

They also had increased brain levels of beta-amyloids -- the proteins that form plaque deposits in the brains of Alzheimer's sufferers.

Alzheimer is the most common form of dementia, a general term for memory loss and other intellectual abilities serious enough to interfere with daily life.

The study has found that stress hormones, which are elevated in the brain when a person is harassed, inhibit brain activity. If the hormone levels are chronically elevated, this can lead to the development of Alzheimer's disease.

Bengtsson believes that the effect of stress hormones on the brain could mean the difference between living independently and needing to be put into care.

However, Simon Ridley, head of Research at Alzheimer's Research UK, said: "It's important to remember this research was not carried out in people.

Some research has already highlighted a possible link between chronic stress, cognitive decline and the development of Alzheimer's, and further study in people is needed to fully investigate these links.

The latest research comes just after another study which suggested disrupted sleep could be one of the first signs of Alzheimer's disease.

Researchers at Washington University found sleep was disrupted in people who had early Alzheimer's disease but did not yet have the memory loss or other cognitive problems characteristic of the full-blown disease.