Children experiencing bad dreams and nightmares persistently may be at risk of developing cognitive impairment (including dementia) and Parkinson's disease in adulthood, finds a study.

Distressing dreams in middle-aged and older adults have previously been associated with an increased risk of developing dementia and Parkinson's.

But the study, published in the eClinical Medicine journal showed that children who experienced persistent nightmares between the ages of seven and 11 were nearly twice as likely to develop cognitive impairment by age 50.

The study is the first to find an association between distressing dreams during childhood and an increased risk of developing cognitive impairment or Parkinson's, said researchers from the University of Birmingham in the UK.

The number of people living with dementia is projected to triple from 50 million to 150 million globally within the next three decades, and the number living with Parkinson's disease is set to reach 14 million by 2040.

Both dementia and Parkinson's cause distress for those affected, and the economic costs of the two conditions combined exceed 1 trillion annually.

"Given that distressing dream frequency is a relatively stable trait from early childhood to middle adulthood, this raises the possibility that having regular distressing dreams as a child may be a very early indicator of dementia and Parkinson's risk," said researchers.

For the study, the team analysed data from 6,991 children, of which 3.8 per cent or 267 developed cognitive impairment or Parkinson's.

Compared with children who never had distressing dreams, children who had persistent distressing dreams had an 85 per cent increased risk of developing cognitive impairment or Parkinson's by age 50.

"Having persistent distressing dreams during childhood may be associated with an increased risk of developing cognitive impairment or Parkinson's in adulthood," said Abidemi I Otaiku from the varsity.

However, Otaiku stressed the need for further studies to confirm the findings and to determine whether treating distressing dreams during early life may lower the risk of dementia and Parkinson's.