Los Angeles: In the first study of its kind, scientists have identified 13 inherited traits related to sleep, wake and activity cycles that are associated with severe bipolar disorder.
Researchers also were able to tie the traits to specific chromosomes, providing important clues to the genetic nature of the disorder, as well as potential new avenues for prevention and treatment.
"We were able to identify 13 sleep and activity measures, most of which are inherited, that correlated with whether an individual had bipolar disorder. In addition, we were able to trace some of these traits to a specific chromosome," said Joseph Takahashi, from University of Texas' Southwestern Medical Centre.
"This study represents a key step in identifying the genetic roots of this disorder and, in turn, providing targets for new approaches to preventing and treating bipolar disorder," said Nelson Freimer, from University of California, Los Angeles.
Bipolar disorder, or manic-depressive illness causes dramatic mood shifts - often called episodes - in which the person is overly excited, extremely sad or depressed, or a mixed state of both, including irritable or explosive behaviour, according to the US National Institute of Mental Health.
Causes of bipolar disorder (BP) are thought to be both genetic and environmental, and researchers have long suspected that disruption in normal daily circadian rhythms, including sleep and wake cycles, can precede mood shifts.
Those with bipolar disorder awoke later and slept longer, on average were awake fewer minutes overall, and were active for shorter periods than those without the disorder.
They also displayed lower activity levels while awake and had greater variations in sleep and wake cycles.
The study involved more than 500 members of 26 families from Costa Rica and Colombia, identified because they each include many individuals with severe bipolar disorder.
It is the first large-scale delineation of sleep and activity traits in BP-affected individuals and their relatives and the first genetic investigation of such a comprehensive set of sleep and circadian measures in any human study.
The study was published in the journal PNAS.