Scientists have reason to believe that the sleeping patterns of teenagers can provide a clue to their longer-term risk of developing depression or bipolar disorder.
Erratic sleeping patterns were an often overlooked feature of "basically all mood disorders and all psychiatric disorders", explained Naomi Rogers of the University of Sydney's Brain and Mind Research Institute.
"In people who develop depression, often you can trace back and find they have had early sleep disturbance," the Daily Telegraph quoted Rogers, as saying.
"We know that disturbed sleep occurs in basically all mood disorders and all psychiatric disorders, and the more disturbed sleep patterns are we tend to see worse mood symptoms.
"But whether (disturbed sleep) is an early sign, or risk factor, we are not yet sure," she said.
Rogers has studied sleep-wake cycles, and circadian rhythms, of young people with depression, bipolar disorder and schizophrenia.
She observed results unlike ordinary adolescent sleep patterns, in which young people typically go to bed late then need to sleep in but are able to do so only on weekends.
Young people with mental illness were more likely to show "no pattern", she said.
She cautioned parents to not look at a child's sleep-wake cycle in isolation, but to also look for evidence of "withdrawing", such as quitting their team sport or not seeing as much of their friends or poorer school grades.
Noticing these early warning signs could lead to an earlier diagnosis of a mental health problem and allow an earlier intervention with milder treatments.
"It's the whole picture of looking at their social interactions, their sleep wake-cycle and mood ... people tend to forget about the sleep part," said Rogers.
Sleep was even more important for children during periods of intense study, as research showed more information was retained after a good night's sleep rather than "cramming" into the early hours.
The findings were discussed at the 22nd Annual Scientific Meeting of the Australasian Sleep Association and Australasian Sleep Technologists Association conference, in Christchurch.