Insomnia and mental disorders are linked. But exactly how is still a mystery

This article is the next in The Conversation’s six-part series on insomnia, which charts the rise of insomnia during industrialisation to sleep apps today. Read other articles in the series here.

The 2004 movie The Machinist gives us a striking depiction, albeit a fictional one, of the psychological effects of chronic insomnia.

When people don’t have enough sleep, their memory and concentration are impaired in the short term. They are also less able to regulate their emotions.

If sleeping difficulties continue, longer-term psychological effects can include anxiety, depression, mania and psychosis. Indeed, Christian Bale’s character in The Machinist has increasingly vivid visual hallucinations and paranoid delusions as his insomnia deepens.

The relationship between insomnia and mental disorders is complex. It’s not just a case of “which comes first, the insomnia or the mental disorder?” Insomnia and mental disorders are interrelated in ways we don’t yet fully understand.

Read more: A short history of insomnia and how we became obsessed with sleep

What is insomnia? Is it a mental disorder?

Insomnia is, by far, the most common disorder of sleep. An estimated 12-15% of Australian adults at any one time meet criteria for insomnia.

People with insomnia have frequent and ongoing difficulties in falling and staying asleep, and/or going back to sleep after early waking. Insomnia not only affects people’s quality of sleep, but how they function the next day.

Many of the factors that trigger insomnia and help maintain its negative long-term effects are psychological or behavioural. These include:

• psychosocial stressors (such as money, work or family problems)

• attentional bias and worry about sleep (the more we think and worry about sleep, the worse it gets)

• poor sleep habits (such as irregular sleep and wake times).

Insomnia is not a mental disorder in the same way that, say, depression and anxiety are mental disorders. Insomnia is a recognised sleep disorder that nonetheless has close links to a wide variety of mental disorders.

Read more: What's insomnia like for most people who can't sleep? You'd never know from the movies

Many people have both insomnia and a mental disorder

Around half of all people diagnosed with insomnia also have an associated mental disorder. The most common ones associated with insomnia are depression, anxiety, bipolar disorder, post-traumatic stress disorder, and substance-related disorders.

We don’t (yet) know why we see such high levels of mental disorders in people with insomnia. But there are several leading theories, including:

• a shared genetic risk for insomnia and a mental disorder. In other words, some people’s genes may predispose them to both conditions

• a shared neurobiological response. How the brain responds to sleep loss may be connected to how systems in the brain control cognition, emotion and reward. Disturbances of these brain functions are implicated in a range of mental disorders

• inflammation and/or dysfunction of the immune system may underlie both insomnia and mental disorders.

Read more: How dangerous is insomnia? How fear of what it's doing to your body can wreck your sleep

Which comes first?

Further complicating the picture is evidence showing insomnia can occur before someone develops a mental disorder, or afterwards. Researchers call this a “bidirectional” relationship.

We can’t say one causes the other. We can only say there is a clear link between them.

This link means diagnosis and treatment of one can have implications for diagnosis and treatment of the other. For instance, if you don’t adequately treat insomnia, this can worsen symptoms of someone’s mental disorder, increasing both the severity and risk of relapse.

A type of talking therapy known as cognitive behavioural therapy for insomnia (CBTi) aims to change the unhelpful thoughts, feelings, emotions and behaviours that contribute to insomnia. And, in many cases, successful treatment of insomnia with CBTi can also treat someone’s mental disorder (and vice versa).

CBTi is particularly effective at treating insomnia plus depression, substance use or post-traumatic stress disorder. But it is less effective at treating insomnia plus psychosis or bipolar disorder.