Sleep patterns can help predict which adolescents might be at higher risk of depression, says Indian-origin researcher Uma Rao from UT Southwestern Medical Centre.
Depressed adults experience rapid-eye-movement (REM) sleep earlier in the sleep cycle than people who are not depressed.
Rao showed that adolescents with a familial risk for depression but without a depression diagnosis experienced shorter REM latency, meaning they reached the REM stage more quickly.
Those adolescents were more likely to develop depression.
"Sleep is probably more helpful in determining who is at risk for developing depression than in being a diagnostic marker for depression since REM latency of those adolescents was shorter before they even developed the illness," said Rao.
Adolescent depression is complex to prevent and to treat in part because baseline levels of sleep and other factors used to diagnosis depression are not clearly defined.
For example, in clinical studies, adolescents without manifestation of mental illness can be labeled erroneously as control group members because they haven't yet reached the highest -risk period for developing depression - mid- to late-adolescence and early adulthood.
"Comparing these younger adolescents to those already showing depression obscures study results and can affect our understanding of the underlying mechanisms for depression as well as its treatment," Rao said.
"This study is an initial step in determining baseline measures that differentiate healthy adolescents from those who are likely to develop depression, bipolar disorder and other mental diseases as they get older," she added.
Researchers also studied another biological factor known to be associated with adult depression - cortisol, a hormone that is increased when humans are under stress.
During the study, the researchers looked at 96 adolescents with no evidence of depression or other psychiatric disorders, researchers monitored the sleep cycles of participants for three days and collected saliva and urine samples to record cortisol levels.
They found that at the end of the five-year study period, adolescents with higher cortisol levels were more likely than others to develop depression.
"Depression is not mediated by sleep alone," Dr. Rao said.
"If we can identify factors such as sleep and cortisol and their role, we could start the prevention process before the disease leads children and teenagers down a path well behind their peers educationally and socially," she added.
The study appears in journal Neuropsychopharmacology.