Bad dreams may cause us to wake in a cold sweat in the night, but it’s not something you would bother mentioning to your doctor. Or should you?

Scientists believe that recurring nightmares could provide vital clues about health and even warning signs about impending illness – sometimes years before symptoms appear.

Last month, researchers found regular nightmares in childhood could be an early warning sign of psychotic disorders later in life. The study, published in the journal Sleep, tracked 6,800 children and found those having frequent nightmares (two to three times a week) between the ages of two and seven were three-and-a-half times more likely to have a psychotic experience, such as hallucinations or hearing voices, as a teenager. 

The researchers said frequent nightmares could indicate children are facing  emotional trauma, such as abuse or  bullying, in waking life. They said use of violent video games and computers near bedtime may also play a part.

It’s thought we may experience terrifying threats in our nightmares to practise handling them in waking life, explains Patrick McNamara, associate professor of neurology at Harvard Medical School. But nightmares may also indicate an underlying physical problem that is disrupting our sleep as we dream. We only dream during the stage known as rapid eye movement (REM) sleep. We experience REM, a very light sleep, on average four to five times a night.

Lots of medical problems cause disruption to sleep, which means you’re more likely to wake during the REM phase and remember that you had a nightmare, says Dr Nicholas Oscroft, a sleep researcher and cardiologist at Papworth Hospital in Cambridge.

Regular nightmares, could, for example, be a sign of sleep apnoea – this causes breathing to stop temporarily as the airways become obstructed. Patients with sleep apnoea often report frequent nightmares.

Regular bad dreams can be linked to heart problems, says Dr Oscroft. People who have regular nightmares were three times more likely to suffer irregular heartbeat, according to a study of more than 6,000 adults, published in the Netherlands Journal of Medicine in 2003. Chest pain was seven times higher in those who reported having nightmares often.

One theory, Dr Oscroft explains, is that people with heart conditions, in particular heart failure (when the heart can’t adequately pump blood to the lungs and other organs), suffer breathing problems at night.

Heart failure leads to a build-up of water in the lungs, which makes breathing more difficult, particularly at night in REM sleep. This is because most of the muscles become paralysed during this stage, to stop us acting out our dreams; but this can affect the breathing muscles. This can wake people during REM sleep, making them more likely to remember bad dreams.

Any kind of infection, from severe flu to a kidney infection, can make nightmares more likely, explains Professor McNamara.

Migraines, too, are often preceded by unpleasant dreams, involving themes of anger, aggression and misfortune, according to a study in Psychotherapy and Psychosomatics in 1996. Around one in four migraine sufferers experiences an aura before an attack: they see flashing lights or zigzags, and hallucinations.

‘People who don’t sleep enough tend to miss out on REM sleep,’ says Dr Oscroft. REM sleep occurs around 90 minutes after we fall asleep, at the end of each sleep cycle; we experience on average four cycles of sleep.

As each period of REM sleep gets longer through the night, we get more in the latter part. If you’re sleep deprived, even by one or two hours a night for a week, when you finally do sleep, you experience REM rebound, getting more dreaming sleep.

This is because your brain, which has been missing out on the REM sleep required to process memories, learning and emotions each day, constantly wants to go back into it. ‘The build-up of unprocessed emotion can lead to more reported nightmares,’ says Professor McNamara.