The development of nightmares later in life could be an early sign of Parkinson's disease, according to new research in older men.

Distressing dreams have long been associated with the neurological disease, especially among men, but this is the first study to investigate whether these symptoms are a warning of Parkinson's or a byproduct of the condition.

Tracking the health of 3,818 older men with typical brain functioning for 12 years, researchers found those who experienced frequent nightmares were twice as likely to develop Parkinson's.

Most of the diagnoses occurred within the first five years of the study.

The results suggest older adults could be screened for Parkinson's by asking them about the content of their dreams. Early interventions could then be employed to help stall the possible onset of physical symptoms, like tremors, stiffness, and slowness.

One of the biggest challenges with Parkinson's disease is early diagnosis. By the time most people figure out they've got the disease, they've already lost between 60 to 80 percent of dopamine-releasing neurons in part of their brain stem.

What's more, a previous study by the same researcher found patients with distressing dreams are five times more likely to show rapid disease progression.

"Although it can be really beneficial to diagnose Parkinson's disease early, there are very few risk indicators and many of these require expensive hospital tests or are very common and non-specific, such as diabetes," explains neurologist Abidemi Otaiku from the University of Birmingham in the UK.

"While we need to carry out further research in this area, identifying the significance of bad dreams and nightmares could indicate that individuals who experience changes to their dreams in older age – without any obvious trigger – should seek medical advice."

The link between sleep and Parkinson's is one that researchers have been investigating for several years now.

Roughly a quarter of Parkinson's patients report frequent distressing dreams from the time of diagnosis, and some report experiencing bad dreams up to 10 years before they were diagnosed.

Past studies suggest that people with Parkinson's disease are four times more likely to experience frequent nightmares than those in the general population.

Parkinson's patients are also more likely to develop rapid eye movement sleep disorders, which cause dreams to be physically reenacted during the night.

Yet until now, it hasn't been clear if these symptoms were a byproduct of Parkinson's or prodromal, which is the term scientists use for minor symptoms that appear before major symptoms arrive on the scene.

The current research helps clear up that distinction by tracking a large sample of older men across more than a decade.

In the study, participants with self-reported frequent distressing dreams were two times more likely to develop Parkinson's over 12 years.

What's more, in the first four years of the study, frequent distressing dreams were associated with a six-fold increase in the risk of developing the neurological disease.

Without further research to measure brain activity during sleep, it's hard to say what's going on at a biological level in Parkinson's patients who experience nightmares.

Men with Parkinson's tend to have more disturbing dreams than women with Parkinson's, but why that is remains unclear.

One hypothesis is that the late onset of nightmares is an early sign of neurodegeneration in some men.

Women are significantly more likely to experience regular nightmares early in life, but after age 65, men start to catch up.

Perhaps something is changing in the frontal cortex, which regulates emotion during sleep, as the male brain ages.

Researchers are now planning to use electroencephalography to figure out what that something might be.

The study was published in EClinicalMedicine.