Insomnia has been considered a symptom of various mental disorders since doctors began to study the workings of the brain. New thinking is reversing the direction of that logic, however. Could sleep disorders be the cause of mental illnesses?
Waking our way to madness
Take anyone with a psychiatric disorder and the chances are they don't sleep well. The result of their illness, you might think. Now this long-standing assumption is being turned on its head, with the radical suggestion that poor sleep might actually cause some psychiatric illnesses or lead people to behave in ways that doctors mistake as the result of mental problems. The good news is that sleep treatments could help or even cure some of these patients. Shockingly, it also means that many people, including children, could be taking psychoactive drugs that cannot help them and might even be harmful.
No one knows how many people might fall into this category. "That is very frightening," says psychologist Matt Walker from the University of California, Berkeley. "Wouldn't you think that it would be important for us as a society to understand whether three per cent, five per cent or fifty per cent of people diagnosed with psychiatric problems are simply suffering from sleep abnormalities?" First, we'd need to know how and to what extent sleep disorders could be responsible for psychiatric problems. In the few years since sleep researchers identified the problem, they have made big strides in doing just that.
Doctors studying psychiatric disorders noticed long ago that erratic sleep was somehow connected. Adults with depression, for instance, are five times as likely as the average person to have difficulty breathing when asleep, while between a quarter and a half of children with attention-deficit hyperactivity disorder (ADHD) suffer from sleep complaints, compared with just seven per cent of other children.
Until recently, however, the assumption that poor sleep was a symptom rather than a cause of mental illness was so strong that nobody questioned it. "It was just so easy to say about a patient, well, he's depressed or schizophrenic, of course he's not sleeping well - and never to ask whether there could be a causal relationship the other way," says Robert Stickgold, a sleep researcher at Harvard University. Even when studies did seem to point in the other direction, the findings were largely overlooked, he says.
In 1987, for example, Patricia Chang and colleagues at Johns Hopkins University in Baltimore reported a study of 1,053 male medical students who had been followed for an average of 34 years after graduation. During that time, 101 of them developed clinical depression and 13 of these committed suicide. It turned out that students who had reported suffering from insomnia were twice as likely to develop depression as those with no trouble sleeping. The team concluded cautiously that insomnia was "indicative of a greater risk" of problems later. Dr Stickgold goes further. He believes the study shows that insomnia can predispose people to depression. He's not the only one to be persuaded by the growing realisation that some sleep problems generate symptoms that mimic those of certain psychiatric disorders.
In 2006, Paul Peppard at the University of Wisconsin-Madison and his team studied the relationship between depression and sleep-disordered breathing. In sleep apnoea, the most common breathing problem, a blockage or narrowing of the windpipe causes a steep drop in oxygen levels, temporarily waking the sleeper. The team randomly selected about 800 men and 600 women from a working population and evaluated them in the lab for breathing disorder and depression. There are four categories of sleep-disordered breathing and for each increase in a person's category - from "minimal" to "mild", for example - their odds of getting depressed almost doubled, the team found. Depression cannot have been the main cause of the poor sleep, which is linked to excess fat thickening the windpipe or a large tongue or tonsils relative to the size of the windpipe opening. Instead, this work suggests that sleep disorders lead to the depression.
Indeed, Daniel Buysse, the medical director of the Sleep and Chronobiology Program at the University of Pittsburgh in Pennsylvania, has found that treating depressed patients' sleep problems with a drug such as benzodiazepine can produce a dramatic turnaround in their mood disorder. Poor sleep may also explain some of the characteristic behaviours associated with other mental illnesses. For example, there is plenty of evidence to suggest that impaired sleep can induce the manic episodes suffered by people with bipolar disorder. Dr Stickgold even thinks that it can cause a common problem associated with schizophrenia, namely, the failure to master rote tasks such as how to use a piece of machinery.
It also seems that behavioural problems resulting from lack of sleep may be misdiagnosed as ADHD. In an analysis of 83 children with ADHD, David Gozal from the University of Louisville, Kentucky, and colleagues found that a quarter of those diagnosed with a mild form suffered from sleep apnoea, compared with just five per cent of those with strong ADHD and five per cent of healthy controls. What's more, a study of children undergoing surgery to remove their tonsils and adenoids (a common treatment for snoring and sleep apnoea) found that before the operation, one-quarter had a diagnosis of ADHD compared to 7.4 per cent of healthy controls. But a year after the operations, half of these children no longer met the criteria for ADHD. Mark Kohler from the Women's and Children's Hospital in Adelaide, Australia, who has studied links between ADHD and sleep, suspects that some children are being treated with drugs such as Ritalin while their true problem, a sleep disorder, goes unrecognised.
So how does poor sleep lead to behavioural and psychological problems? Some of the links are apparent. For example, every parent knows that tired children usually become hyperactive rather than sleepy. Sleep disruption also bumps up stress hormone levels, which could contribute to daytime anxiety, a component of many psychiatric disorders. More intriguingly, it now seems sleep disruption can fundamentally interfere with the brain's ability to process emotion. However, there is still a lot of work to be done in untangling the ways in which sleep disruption might create psychiatric symptoms.