Now here’s an interesting one – people who have nightmares in middle age run the risk of dementia in later life.

This surprising news comes from a Birmingham University study and suggests nightmares may be an early warning sign of dementia long before it sets in.

Dr Abidemi Otaiku, of the university’s Centre for Human Brain Health, said: “We’ve demonstrated for the first time that distressing dreams, or nightmares, can be linked to dementia risk, and cognitive decline among healthy adults in the general population.

“This is important because there are very few risk indicators for dementia that can be identified as early as middle age. While more work needs to be done to confirm these links, we believe bad dreams could be a useful way to identify individuals at high risk of developing dementia, and put in place strategies to slow down the onset of disease.”

The study examined data from three community-based groups in the US including more than 600 adult men and women aged between 35 and 64, and 2,600 adults aged 79 and older. No one had dementia at the start of the study and people were followed up for an average of nine years for the younger group and five years for the older.

The researchers started collecting data between 2002 and 2012 when participants completed a range of questionnaires, including the ­Pittsburgh Sleep Quality Index, which has a question on how often people have dreams.

Analysis of the data aimed to find out whether participants with a higher frequency of nightmares were more likely to go on to develop cognitive decline and be diagnosed with dementia.

The results show middle-aged people (35-64) who have bad dreams once a week are four times more likely to experience cognitive decline over the following 10 years, while older people are twice as likely to be diagnosed with dementia.

The study revealed this link was much stronger for men than women. For example, older men having nightmares on a weekly basis were five times more likely to develop dementia than older men reporting no bad dreams. In women, however, the increase in risk was less than half.

The researchers are now going to investigate whether nightmares in young people can foretell future risk of dementia. How often we remember dreams and how vivid they are could also be used as signals of dementia risk.

Using EEG (electroencephalography) and an MRI (magnetic resonance imaging), the researchers plan to investigate what could be at the root of bad dreams in both healthy people and people with dementia.