Studies in this maternal behavior may be motivated in part by claims that bed sharing is a significant independent risk factor for the Sudden Infant Death Syndrome (SIDS). Mother-infant bed sharing may also impact other types of infant health issues, apart from SIDS. Despite this concern, many mothers believe that the infant's sensory and social environment during bed sharing is enriched compared to that found during solitary sleeping.
The study of mother-infant bed sharing is still a nascent area of research; therefore the actual risks or benefits of bed sharing on infant health and development are largely unknown. Since so many aspects of infant physiology and behavior are subtly influenced by the presence of the mother during sleep, any one effect, considered in isolation, could be seen as clinically beneficial, deleterious or neutral. Of course, the overall effect of bed sharing on the infant could be completely opposite to the effect of any one physiological or behavioral response.
As data accumulates on the various physiological effects of bed sharing on infant physiology, more specific explanations will emerge of the way the mother's presence during sleep affects her infant. For example, increases in heart rate could be an indication of increased sympathetic activity, possibly resulting from some specific feature of the bed sharing sensory or psychological environment. In addition, it is almost a certainty that primary physiological effects of bed sharing elicit secondary effects that may impact the infant: data suggests that the increase in body temperature noted in bed sharing infants may be due to increased movements during sleep in those infants.
Since external sensory input is generally arousing and involves increased sympathetic activity, California physiologists hypothesized that infant heart rate would be increased during mother-infant bed sharing. Since there was no evidence for or against an effect of external sensory input or psychological effect on heart rate variability, they proposed the null hypothesis that variability was the same between the two environments.
A New Study
The authors of "Mother-Infant Bed Sharing is Associated With an Increase in Infant Heart Rate" are Christopher A. Richard, Ph.D., of the Department of Neurobiology, University of California, Los Angeles, and the Sleep Disorders Center, St. Joseph's Hospital, Orange, CA; and Sarah S. Mosko, Ph.D., also of St. Joseph's Hospital. These studies were done from 1993-1999 at the University of California, Irvine Sleep Disorders Center. They are presenting their findings at the upcoming meeting, Experimental Biology 2003, co-sponsored by the American Physiological Society (APS). Some 8,000 attendees are expected to gather at the San Diego Convention Center, San Diego, CA, April 11-15, 2003.
Methodology
Thirty-five healthy mother-infant pairs were recruited for a three-night study in the sleep laboratory for polysomnography (sleep study) with video recording. In 15 of those pairs, the ECG was digitized and used to derive heart rate and constitutes the sample in this study. All mothers were interviewed to determine their normal sleeping arrangements at home and those that consistently practiced either bed sharing or solitary sleeping were assigned to routine bed sharing or routine solitary sleeping groups: pairs that used both practices were excluded from the study. All the infants were full-term and between 11 and 15 weeks old.
Each mother-infant pair slept the first or "adaptation" night in the sleep laboratory in their routine home condition. Subsequently, pairs spent one night in their routine condition and one night in the other sleeping condition, in random order. The three-night protocol allowed a 2x2 repeated-measures statistical analysis (ANOVA). The within-subject comparison was between the bed sharing night and the solitary night for both groups, allowing an assessment of the acute or immediate effects, while the across-subject comparison was between routine bed sharers and routine solitary sleepers on both nights to assess long-term, possibly developmental, effects.
This study involved standard polysomnographic paper recordings (i.e. EEG, eye movement, and muscle activity) and sleep stage scoring. The variables tested were median heart rate and the interquartile range of heart rate, as a measure of variability. Separate analyses were performed for data from awake, two categories of non-REM (quiet) sleep and REM sleep.
Results
Mother-infant bed sharing affected heart rate and variability in a state-specific manner. ANOVA analyses showed a significant night effect where heart rate during bed sharing was higher than during solitary sleeping in all sleep stages but not waking. There were no significant long-term effects (routine bed sharers vs. routine solitary sleepers) or interactions between group and night in any sleep/wake state. On average, compared to the solitary night, heart rate on the bed sharing night was 8.8 percent higher in light non-REM sleep, 9.2 percent higher in deep non-REM sleep and 5.5 percent higher during REM sleep. Regression analysis of heart rate with the other physiological and behavioral variables affected by bed sharing in these same infants suggest that changes in body temperature may be a factor in the higher heart rates.
Analyses of the infants' interquartile ranges showed a significant night effect for light non-REM sleep and for REM sleep but not for deep non-REM sleep or for waking In both of the affected sleep stages; the bed sharing night was associated with a reduction in variability compared to the solitary night. Although not significant, the pattern in deep non-REM sleep was similar to that in light non-REM sleep and REM. When the changes in basal heart rate were factored into the variability analysis, only the effects in light non-REM sleep persisted.
Identical analyses of the mothers' heart rate data revealed a significant immediate effect for light non-REM sleep where the bed sharing night was associated with a slower heart rate than the solitary night. In addition, there was a significant group effect during deep non-REM sleep with lower heart rates in the routine bed sharers than in the routine solitary sleeping mothers. The variability in the mother's heart rate was not significantly affected by bed sharing.
Conclusions and Comments
The results of this study indicate that the changes in heart rate reflect the immediate effects of the sleep environment and are not due to long-term or developmental consequences of the choice of sleeping condition. These effects on heart rate and variability are likely related to sensory or psychological stimuli unique to the bedsharing environment, or conversely, to the lack of those stimuli in the solitary condition. More specifically, environmental effects on infant body temperature may be related to the differences in cardiovascular activity and in a sleep-state dependent manner. The clinical significance of the differences in heart rate and its variability between the two sleeping environments is not clear since clinical outcomes were not a part of the experimental design. However, it does appear that the bedsharing infants exhibit an increase in sympathetic activity, compared to solitary sleeping infants.
While there is strong epidemiological evidence that bed sharing is not an independent risk factor for SIDS, it is interesting that bed sharing increases the SIDS risk related to maternal smoking and that the expression of that increased risk may be related to deficiencies in arousal which are accompanied by abrupt increases in sympathetic activity.
Until other studies are available, it would be premature to conclude that any positive or negative clinical effects are associated with the heart rate changes reported here.
The American Physiological Society (APS) is one of the world's most prestigious organizations for physiological scientists. These researchers specialize in understanding the processes and functions underlying human health and disease. Founded in 1887 the Bethesda, MD-based Society has more than 10,000 members and publishes 3,800 articles in its 14 peer-reviewed journals each year.
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