The study also determined whether it was unsettled sleep patterns in infants that affected their parents/carers or whether the quality of parents' relationships affected the child's sleep patterns.
What they found surprised the team- instability in a parents' relationship when the infant was nine months old still affected the child when s/he was 18 months old.
The study was conducted by researchers at the Oregon Social Learning Center, University of Leicester, Cardiff University, University of Pittsburgh, University of California at Davis, The Pennsylvania State University, University of New Orleans, and Yale Child Study Center. The findings appear in the journal Child Development.
Professor Gordon Harold, of the School of Psychology at the University of Leicester, said: "Regulated sleep is essential during infancy for healthy brain and physical development. Disrupted sleep patterns early in life have serious implications for children's long-term development.
"How couples/parents relate to each other, specifically how they manage conflicts in their everyday lives, is also recognized as having significant implications for children's long term emotional, behavioural and academic development.
"Understanding which comes first, children's sleep problems affecting parent relationship quality or parent relationship quality affecting children's sleep problems has significant clinical implications."
Using a sample of over 300 children and their caregivers (mothers and fathers) in the United States, the association between infant sleep problems and parent's relationship instability was examined when children were 9 months and 18 months. Results were such that parents' relationship instability at 9 months predicted infant sleep problems at 18 months, but not the other way around.
"So, how parents relate to each other affects children's sleep patterns, rather than sleep patterns affecting parents' relationship quality; thereby significantly informing our understanding of early family relationship influences on children's development," said Professor Harold who is Chair in Behavioural Genetics and Developmental Psychopathology at the University of Leicester.
A most important and unique feature of the study was that the children and caregivers who took part in the study were adoptive children and parents, with all children in the study adopted at birth. Professor Harold said this is a very important design feature of the study in that it allows the researchers to examine whether another possible explanation for any association between family relationship influences, such as couple relationship instability, on child behavior, such as disrupted sleep patterns, may be a product of shared genetic influences between parents and children.
He said: "When parents and children are biologically related, any association between how parents behave and attributes of child behavior may be explained by common genetic factors (same genes underlying parent and child behavior). The present study rules this explanation out in that parents/caregivers and children are not genetically related, so common genetic factors cannot account for the associations noted. This study does not negate the importance of genetic (nature) and biological factors underlying children's development but does locate the dynamic between couples as a unique family "environmental" influence (nurture) on children's early development."
The study was initiated while Professor Harold was at Cardiff University and finalized while at the University of Leicester.
The study was funded by the Eunice Kennedy Shriver National Institute of Child Health and Human Development, the National Institute on Drug Abuse, and the Office of the Director of the National Institutes of Health.
Source: Leicester University