Young women with regular sleep problems have a four to fivefold increased risk of depression, a landmark study of nearly 10,000 Australians has found.
Experts say sleep disorders could be the hidden cause of depression in a huge proportion of young people, with changes in sleep behaviour potentially reaping benefits in physical and mental health down the track.
The study of women aged between 21 and 25 found about one in 10 often experienced problems sleeping, but otherwise had no symptoms of mental illness. But nine years later those women were four to five times more likely than others in the study to be diagnosed with depression, according to a presentation by psychologist Dorothy Bruck at the Australian Psychological Society's inaugural Health Psychology conference on Friday.
The executive director of the brain and mind research institute at the University of Sydney, Ian Hickie, said focusing on disruptions to people's sleep cycles, or circadian rhythm, could help identify previously elusive causes and treatments for major depression.
''It could be that these are not really mood disorders, they are really brain-clock disorders, and mood is really road kill on the journey,'' he said.
So-called ''atypical'' depression involving sleep disorders could be found in up to 40 per cent of cases, and could potentially put people at risk of bipolar disorder too.
Professor Hickie said young people with depression linked to sleep cycles could react badly to treatment with antidepressants, which have a negative effect on circadian rhythms.
''I think that's why in teenagers it's been so hard to work out if antidepressants are good or bad,'' he said. ''The typical group does very well on them but the atypical group probably does very badly.''
People with low levels of the sleep hormone melatonin were also at risk of weight gain and diabetes, meaning medications linked to those problems were particularly risky.
Professor Hickie said it remained to be seen whether changing people's behaviour through training on sleep and wake routines could prevent them developing depression.
''If you stay up later and later and later, you have got a problem,'' he said. ''You have to go to bed by midnight, no matter which way you cut it.''
Professor Bruck, a professor of psychology at Victoria University, said sleep difficulties could affect brain function, leading to depression.
''When you have sleeping loss that's chronic and severe, it actually undermines your emotional regulation the next day,'' she said. ''I see people all the time who have insomnia, and I can see very much how that insomnia undermines their ability to regulate their emotions.''
Professor Bruck, who is also a director of the Sleep Health Foundation, said her results underlined that people needed to take sleep seriously, although not become overly anxious.
''Often just people learning about sleep can actually decrease their anxiety about sleep and give them better sleep,'' she said. ''But people with sleep difficulties often put a lot of pressure on themselves, and they're often perfectionists.
''They expect things to work straight away but I tell them it's like working out at the gym: you don't see benefits straight away.''
She found sleep problems were particularly persistent, with those suffering from them at the beginning of the study being nearly 12 times more likely to experience them nine years later.