Tuesday January 22 5:21 PM ET

Babies' Sighs During Sleep 'Reset' Nervous System

By Kathleen Doheny

RANCHO MIRAGE, California (Reuters Health) - Parents who hear their sleeping infants sigh from time to time should be reassured, not alarmed. Sighs are an indication of healthy sleep and sometimes serve as an important survival mechanism, Belgian researchers reported here Friday at a pediatric sleep meeting.

When a sleeping infant emits a sigh--defined in sleep researchers' terms as a breath with at least twice the amplitude of the preceding one--the sigh seems to reset the autonomic nervous system, which controls blood pressure, heart rate, body temperature and other important bodily functions, according to Dr. Sonia Scaillet of University Children's Hospital, Brussels, the study's lead author.

The autonomic nervous system has two components, which act in balance with one another. One component, the sympathetic nervous system, takes over during stressful situations, for example when a person is running from danger. The other, the parasympathetic nervous system, takes control during less stressful times. A baby's sigh, the researchers found, acts like a ``reset button'' to rebalance the two systems.

The team analyzed 107 sighs isolated from sleep recordings of 23 full-term infants, whose median age was 33 weeks. They compared the 2-minute period before the sigh with the 2-minute period after. Before the sigh, the investigators found, babies' sympathetic activity was decreasing, while their parasympathetic activity was on the rise. After the sigh, the opposite took place: parasympathetic activity dropped, while sympathetic activity rose.

``The sigh is a healthy mechanism,'' said Dr. Patricia Franco, who presented the team's research at the 20th Annual Conference on Sleep Disorders in Infancy and Childhood. ``If there is a problem with breathing, the sigh can be important for oxygenation,'' she told Reuters Health.

Their finding ``indicates that sighs could play a 'resetting' role for the autonomic system during quiet (deep) sleep in infants,'' the authors write in their conclusion.

``Some other groups have looked at sighs in relation to heart rate, lung function and arousal reactions,'' Scaillet told Reuters Health. ``But to my knowledge, we are the first to study the relationship between sighs and the autonomic nervous balance.''

Recently, many sleep researchers have focused on arousal mechanisms such as sighs, Franco said, in efforts to learn more about pediatric sleep and sleep disorders, such as sudden infant death syndrome (SIDS).

But to say that an infant who sighs is minimizing his risk of SIDS is definitely a stretch, Scaillet added.

``Sighs have been shown to be part of the arousal reaction, and failure to arouse has been implicated in SIDS. So, a child who sighs probably possesses adequate arousal mechanisms,'' she noted. ``But that in itself may not minimize the risk of SIDS, as environmental factors (smoking, sleeping position, room temperature, among others), sickness and maturation have all been implicated in SIDS.''

For now, parents should be aware that a sigh is a normal component of breathing, the researchers conclude. ``Sighs are one mechanism of survival,'' Franco said.

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