A U.S. researcher found the mothers of twins sleep an average of 5.4 hours in a 24-hour period, rather than the recommended 7-8 hours. Almost half of the mothers reported mild to severe symptoms of depression.

"As primary caregivers for families, mothers caring for twins experience enormous workload, extreme exhaustion and limited time to meet their own needs," study author Dr. Elizabeth Damato of Case Western Reserve University in Cleveland said in a statement.

"Additionally, mothers of twins are likely to be caring for babies that are premature ... Meeting the increased demands of two premature infants places mothers at risk for sleep deprivation."

Recent evidence suggests sleep deprivation and the resultant fatigue are related to the development of postpartum depression, Damato said.

"Postpartum depression is known to have negative effects on the quality of mother-child interactions and on the child's biological and behavioral development."

The study focused on 14 mothers of twins that were, on average, delivered 3 1/2 weeks early.

The findings were presented at the 22nd annual meeting of the Associated Professional Sleep Societies in Baltimore.