A groundbreaking new study tracking almost 2,000 young Australians for a decade has challenged a long-held assumption about depression and physical health, finding disrupted sleep was a stronger predictor of later insulin resistance rather than weight gain.
The research from the University of Sydney's Brain and Mind Centre found that weight gain is not the main reason depression is linked to later insulin resistance, which is an early marker of diabetes and heart disease.
Instead, the research points to irregular sleep-wake cycles and ongoing sleep disruption as a key and often overlooked pathway connecting mental health issues in youth to physical illness years later.
Professor Ian Hickie from the Brain and Mind Centre said the results pointed to a significant missed opportunity for early intervention.
"Weight didn't explain the link between depression and later metabolic risk, the signal was already there years earlier, and it was evident in disrupted sleep," Professor Hickie said.
"The physical health consequences of depression may start much earlier than we realise, and in ways that aren't visible or noticeable at first. One of the earliest signs is disrupted sleep-wake cycles. If we wait until people gain weight or develop diabetes, we may have missed a critical window for intervention among young adults."
Dr Jacob Crouse, a Senior Research Fellow at the Brain and Mind Centre, said that such sleep disturbances can be improved with simple lifestyle changes.
"There are easily implementable behavioural changes we can all make that have powerful effects on how well we sleep and how our biological clocks function, like waking up at the same time each morning, getting exposure to sunlight every day (via walking, exercising, or simply sitting by a window), and turning the lights down when the sun sets. Such strategies might have important flow-on effects for our mental and metabolic health."
The study followed more than 1,700 Australians over a decade from their early 20s into their 30s and is one of the first of its kind to track this link in a community-based sample, before obesity or chronic illness typically appear.
The findings show that metabolic risk, which include insulin resistance, poor blood sugar regulation and increased risk of diabetes and cardiovascular disease, can begin silently in young people who show no obvious signs of ill-health.
Key findings:
Depression in early adulthood was linked to the development of ongoing sleep-wake disruption over time
The sleep disruptions, not weight gain, were the stronger predictor of insulin resistance a decade later
The link was present even in young people who were not overweight and had no obvious physical illness
Because the sample was community-based rather than clinical, the findings suggest a population-level connection between mental and physical health that extends well beyond severe cases
Current approaches to preventing chronic disease linked to mental ill-health tend to focus on weight management and lifestyle interventions, but the study suggests at that point it is too late.
"Focusing only on weight misses part of the picture," Professor Hickie said.
"Sleep and daily rhythms deserve much more attention in mental health care, and early support in young adulthood could reduce the burden of physical disease decades later."
The study can be found in the Journal of Affective Disorders.
