Inappropriate amounts of sleep may activate depression-related genes, researchers report in the issue of the journal Sleep.
Read: Depression and sleep
One study included more than 1 700 adult twins. Among those who got normal amounts of sleep (seven to nearly nine hours a night), the genetic influence on symptoms of depression was 27% versus 53% for those who slept only five hours a night, and 49% among those who slept 10 hours a night.
"Both short and excessively long sleep durations appear to activate genes related to depressive symptoms," lead investigator Dr Nathaniel Watson, an associate professor of neurology and co-director of the University of Washington Medicine Sleep Centre in Seattle, said in a journal news release.
Ensuring that patients get optimum levels of sleep may be one way to boost the effectiveness of treatments for depression, he added.
The second study included more than 4 100 youngsters aged 11 to 17. It found that sleeping six hours or less per night increased their risk for major depression, which in turn increased their risk for too little sleep.
"These results are important because they suggest that sleep deprivation may be a precursor for major depression in adolescents, occurring before other symptoms of major depression and additional mood disorders," principal investigator Dr Robert Roberts, professor of behavioural sciences in the School of Public Health at the University of Texas Health Science Centre in Houston, said in the news release.
"Questions on sleep disturbance and hours of sleep should be part of the medical history of adolescents to ascertain risk," he added.
"Healthy sleep is a necessity for physical, mental and emotional well-being," Dr M. Safwan Badr, president of the American Academy of Sleep Medicine, said in the news release. "This new research emphasizes that we can make an investment in our health by prioritising sleep."