Dreams and nightmares are a topic that have intrigued and puzzled scientists for centuries.
While they have been the subject of intense research, no one is certain why we actually dream, and opinions are split on what dreams can mean.
Dreams range from the mundane – most of us have dreamt we’ve got up and got ready for work, only to wake up and realise we have to do it all over again – to the surreal, while nightmares can have both children and adults waking up in a cold sweat.
Here’s what experts can tell us about why we experience nightmares, and when we are most likely to have them.
What are nightmares?
According to the Harvard Medical School, “nightmares are simply dreams that cause a strong but unpleasant emotional response”.
Most dreams and nightmares that we recall happen during REM sleep – a stage of sleep that is characterised by rapid eye movement, irregular heartbeat, and increased rates of respiration – that makes up roughly 20 per cent of our night’s sleep. We tend to have four or five short periods of REM sleep each night.
Nightmares are most likely to happen during the period of sleep when REM intervals lengthen, which usually occur halfway through your sleep.
Harvard Medical School explains: “As we prepare to awaken, memories begin to integrate and consolidate. We dream as we emerge from REM sleep. Because we tend to dream on the sleep-wake cusp, images imagined while dreaming, including the vivid, often terrifying images produced during nightmares, are remembered.”
Researchers are split on why we actually dream. The neurologist Sigmund Freud believed dreams were a way for the mind to release repressed thoughts and feelings, while some believe they have no meaning at all.
Why do we have nightmares?
There are plenty of reasons why nightmares can occur, such as stress, anxiety, irregular sleep, medication, mental health disorders, and even reading a scary book or watching a horror film before bed.
One of the most well-researched causes of nightmares is post-traumatic stress disorder (PTSD). Nightmares are one of the criteria used for the diagnosis of the disorder.
A University of Pittsburgh School of Medicine study, published in 2009 in Sleep Medicine Clinics, found that 80 per cent of people with PTSD experience frequent nightmares.
The National Centre for PTSD of the US Department of Veterans Affairs says about half of the people who have nightmares after a traumatic event experience nightmares that replay the trauma, while those with PTSD are significantly more likely to have exact replays of the traumatic event.
Away from PTSD, nightmares tend to be more common in children than adults.
Dr Deirdre Barrett, assistant clinical professor of psychology at Cambridge Health Alliance and editor of Trauma and Dreams, explains: “We think that some of this may be evolutionary. Children are smaller and are vulnerable to many more threats than adults. Nightmares may partially reflect this vulnerability.”