NEW YORK (Reuters Health) ¿ Pre-schoolers' odds of having nightmares may be related to their temperament as infants, which may be noticed as early as five months old, new research suggests.

In a study that followed 987 children from infancy to age six, Canadian researchers found that the majority had an occasional bad dream, while a few had them frequently. The odds of having nightmares ¿ and of having them consistently through the pre-school years ¿ were higher among children who were considered to be more anxious or "difficult" as babies.

The findings suggest that young children's bad dreams "are trait-like in nature and associated with personality characteristics measured as early as five months," the researchers report in the medical journal Sleep.

A previous study with identical and non-identical twins suggested that people may inherit a certain vulnerability to having nightmares, Dr. Tore Nielsen, one of the researchers on the new study, told Reuters Health.

In this study, "bad dreams" as early as the age of 2.5 were predicted by signs of anxiety at the ages of five months and 17 months, explained Nielsen, a professor of psychiatry at the University of Montreal.

Environmental stressors ¿ starting school for the first time, for instance ¿ may conspire with an inherited vulnerability to spur young children's nightmares, according to Nielsen.

The study found that the percentage of children having an occasional bad dream held steady from the age of 29 months to age six ¿ about two thirds at each age, according to parents. Similarly, less than two percent of children had frequent nightmares at each age.

When the researchers looked at mothers' reports on their children's temperaments during infancy, they noted differences between children who had no bad dreams and those who had them consistently through early childhood.

Children in the latter group tended to be more restless and cry more at the age of five months, and they were more difficult to calm at 17 months, according to mothers' reports.

The results raise the possibility that calming infants' persistent distress may relieve them of some bad dreams later in childhood, according to the researchers. In this study, certain parenting routines ¿ like taking two-year-olds out of bed to comfort them when they were distressed ¿ were related to a lower risk of nightmares later in childhood. The study did not examine the effectiveness of any tactics for easing infants' and young children's anxiety ¿ or their influence on the odds of having nightmares later, Nielsen said. However, he added that based on other research, a good starting point would be to improve children's early bonding, or "secure attachment'', with their parents.

For older children who are having distressing dreams, Nielsen said he and his colleagues have found that having the children "draw the dream" and share it with their parents can be helpful.