Positive experiences can protect against the negative impacts bad sleep can have on a person's mental health, US research suggests.
While poor sleep is both a risk factor and a common symptom of depression, not everyone who tosses and turns at night becomes depressed, say Duke University neuroscientists.
People whose brains are "more attuned" to rewards may be protected against the depressive effects of poor sleep.
A study, published in the Journal of Neuroscience, found university students with poor quality of sleep were less likely to have symptoms of depression if they also had higher activity in the brain's reward centre known as the ventral striatum.
"It is almost like this reward system give you a deeper reserve," said Ahamd Hariri, a professor of psychology and neuroscience at Duke University.
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"Poor sleep is not good, but you may have other experiences during your life that are positive. And the more responsive you are to those postitive experiences, the less vulnerable you may be to the depressive effects of poor sleep," Prof Hariri said.
Researchers examined the ventral striatum (VS) of more than 1000 students. The VS helps regulate behaviour in response to external feedback.
Each study participant completed a series of questionnaires to evaluate sleep quality and depressive symptoms, and also completed an MRI brain scan while engaging in a task that activates the VS.
In the task, students were shown the back of a computer-generated card and asked to guess whether the value of the card was greater than or less than five. After they guessed, they received feedback on whether they were right or wrong.
But the game was rigged, so that during different trials the students were either right 80 per cent of the time or wrong 80 per cent of the time.
To tease out whether general feedback or specifically reward-related feedback buffers against depression, the researchers compared VS brain activity during trials when the students were mostly right to those when they were mostly wrong but still received feedback.
They found that those who were less susceptible to the effects of poor sleep showed significantly higher VS activity in response to positive feedback or reward compared to negative feedback.
"Using data from a large sample of 1129 university students we demonstrate that as reward-related VS activity increases, the link between sleep disturbances and depression decreases," the authors wrote.
"This finding contributes to accumulating research demonstrating that reward-related brain function may be a useful biomarker of relative risk for depression in the context of negative experiences."
Professor Hariri hopes the finding may one day help identify people for whom good sleep habits may be more effective or more important.
"This helps us begin to understand why some people are more likely to experience depression when they have problems with sleep," said Prof Hariri.
