Children who were born at a relatively small size may be more likely than their peers to have sleep difficulties, a new study suggests.

The study, which included 289 8-year-olds born healthy and full-term, found that the lower the children’s weight and length at birth, the greater their odds of having poor sleep or sleep disturbances such as sleep-related breathing problems or nightmares.

What’s more, mothers’ prenatal drinking - a habit that can impair fetal growth and development - was linked to a greater risk of childhood sleep problems.

It’s possible that in some children, smaller birth size is a marker of alterations in nervous system development, which might affect the body’s sleep regulation later in life, the researchers report in the journal Sleep.

“We showed that even within children born healthy and at-term gestation, smaller body size at birth increases the risk for poor sleep,” lead researcher Dr. Anu-Katriina Pesonen, of the University of Helsinki in Finland, told Reuters Health in an email.

That does not mean, however, that every child born at a relatively small size is destined for sleep difficulties. For one, birth size is partially determined by genetics, and Pesonen said it’s possible that the current findings do not pertain to newborns who are simply naturally on the smaller size.

On the other hand, she said, factors that can impair normal fetal growth - which, besides prenatal drinking, include prenatal smoking and high chronic stress levels - may help set the stage for sleep problems later on.

For their study, Pesonen and her colleagues had each child wear an actigraph - a watch-like device that measures sleep and activity patterns - for one week. Their parents also completed a standard questionnaire on childhood sleep disturbances.

In general, the researchers found, the smaller a child was at birth, the greater the likelihood of sleep disturbances or low sleep efficiency.

Sleep efficiency refers to a person’s ability to fall asleep and stay asleep once the head hits the pillow. In this study, 26 children had low sleep efficiency - spending roughly three-quarters or less of their time in bed actually asleep.

The researchers also found an association between sleep problems and mothers’ prenatal drinking, even at relatively moderate levels.

Among children whose mothers had consumed more than one drink per week during pregnancy, the risks of short sleep duration - less than 7.5 hours per night - and low sleep efficiency were about three times higher compared with other children.

Prenatal smoking was not linked to sleep problems. However, Pesonen’s team notes, this could be because few mothers in the study said they had smoked during pregnancy, limiting the study’s ability to find an association.

For now, the researchers conclude, the findings suggest that even moderate levels of drinking during pregnancy, and birth size variations within the normal range, may affect children’s sleep later on.

SOURCE: Sleep, August 1, 2009.