Thrashing about in your sleep could mean you have a higher risk of developing dementia or Parkinson's disease, experts say.
Kicking and punching at night is characteristic of a condition called REM sleep behaviour disorder.
Academics say 80 per cent of those who have a severe case of this eventually develop a neurological disease.
The restless behaviour may predict dementia and other diseases up to 15 years before symptoms appear.
Those with sleep behaviour disorders often toss and turn at night, meaning they feel bleary-eyed and fall asleep in the daytime.
In some cases their movements are so vigorous that they can hurt themselves or their partners.
Dr John Peever, of the University of Toronto, who presented his research at the Canadian Association for Neuroscience annual conference yesterday, thinks sleep disorders are caused by dysfunctional brain stem cells.
When most people dream, if these cells are fully functioning, the body is paralysed' so we do not act out the scenes being played in the mind.
The cells called REM-active neurons switch on when the body enters a state of deep sleep known as REM or rapid eye movement sleep, when the eyes dart around beneath the eyelids. This is when we tend to experience vivid dreams.
But for those with sleep disorders these cells never switch on, meaning the body moves around when we dream.
Dr Peever thinks these dysfunctional cells also play a role in Parkinson's, dementia, and other brain and nervous system problems in the elderly. He said: "When we switch on these cells, it causes a rapid transition into REM sleep.
"Our research suggests sleep disorders may be an early warning sign for diseases that may appear some 15 years later in life." Dr Peever hopes his research will eventually allow doctors to predict which patients are likely to develop the diseases.
Knowing someone is at risk may help them adopt a healthier lifestyle, which could delay onset of the illnesses.
Dr Peever added: "Much like we see in people prone to cancer, diagnosing REM disorders may allow us to provide individuals with preventative actions to keep them healthy long before they develop these more serious neurological conditions.
"This goal will take years to develop yet, but could one day help thousands of people live healthier lives long before they need serious medical attention."
In a separate study, teenagers who use mobile phones at night were found to be more likely to suffer poor sleep and be depressed.
Parents should consider 'digital curfews' in the evening, according to the Australian researchers from Murdoch and Griffith universities.
The three-year study of 1,101 teenagers, published in the journal Child Development, also found that those who used phones after nightfall tended to have lower self-esteem and be more aggressive.