Subjects studied are 500 pairs of twins in the same egg or different eggs in Montreal (Canada), born between 1995 and 1998.
The researchers suggest that mothers knew day and night of sleeping time if the child is 6 months old, 18 months, 30 months and 48 months old.
After comparing the twins, Dr. Jacques Montplaisir at the University of Montreal (Canada) and colleagues can determine the difference in a child's sleep-wake rhythm is the result of genetic or environmental.
They conclude, genetic influence is from 47 to 58% of the length of the child sleep at night, while environmental factors influence from 33 to 79% of children sleep on day. This is especially more apparent as the child grows.
The study was published in Pediatrics.