Lack of sleep really can affect a person’s sanity, new research suggests.
A study of healthy volunteers and psychiatric patients showed a clear link between insomnia and paranoid delusions.
Experts found that 70% of ordinary people who scored highly for self-reported symptoms of paranoia had difficulty sleeping.
More than 50% of the psychiatric patients, who all experienced feelings of persecution, suffered from moderate to severe insomnia.
Dr Daniel Freeman, from the Institute of Psychiatry at King’s College London, who led the research, said: "As most of us know, a few nights of poor sleep can make us feel stressed, muddled in our thinking, and disconnected from the world.
"These are ideal conditions for paranoid fears to take hold. Regular, good quality sleep is important to our psychological well -being."
Insomnia is known to cause stress, anxiety and depression, which are themselves associated with paranoid thoughts.
Dr Freeman’s team looked for evidence that lack of sleep contributes to "persecutory ideation" – fantasies of being victimised and picked on.
A total of 300 people drawn from the general population and without histories of mental illness were asked to complete questionnaires designed to highlight symptoms of insomnia and paranoia.
Similar questionnaires were completed by the 30 psychiatric patients, who were all diagnosed with psychotic conditions marked by paranoid delusions.
The scientists wrote in the journal "Schizophrenia Research": "The results were clear: higher levels of insomnia were associated with higher levels of persecutory thinking."
Dr Freeman said the research, funded by the Wellcome Trust, suggested that tackling an inability to sleep could reduce the risk of paranoia.
"The good news is that there are several, tried-and-tested ways to overcome insomnia," he said.
"In particular, cognitive behaviour therapy (CBT) has proven benefits. The intriguing implication of the research is that use of the sleep techniques may also make us feel safer and less mistrustful during the day.
"A good night’s sleep may simply make us view the world in a much more positive light."
Shakespeare was a step ahead of the psychiatrists when he highlighted the link between insomnia and paranoia in the play Macbeth.
After committing the famous murder of Duncan, King of Scotland, Macbeth is plagued by insomnia. Leaving Duncan’s chamber, he thinks he hears a voice cry "sleep no more: Macbeth doth murder sleep".
As the play develops, he becomes increasingly paranoid, carrying out a series of further murders to protect himself against imagined threats.
(Source: Schizophrenia Research: Mental Health Foundation UK: December 2008)