Scientists’ findings may lead to better understanding of how that sleep develops, matures

As any new mother knows, getting a baby to sleep at night is an art, and perhaps using snippets from Shakespeare’s Hamlet may help. But the science of how babies sleep – and what their processes may have in common with their adult counterparts – could be a small step closer to being better understood.

Background

Sleep shows dramatic changes across early development. Quiet sleep (also known as non-rapid eye movement sleep [QS/NREMS]) increases in the course of the first year of life while active sleep or rapid eye movement sleep (AS/REMS) decreases. Slow wave sleep becomes predominant in the first part of the night beginning at about two months of age. In the last 30 years, researchers have increased their knowledge about the quantitative characteristics of the sleep EEG (electroencephalogram) during development, but how sleep regulation develops in early life was not a focus of their work. Accordingly, a new study of human infants has been conducted to document changes of the sleep EEG, with a particular focus on those facets that are markers of the sleep process in adults. The results suggest that EEG markers of sleep homeostasis appear in the first postnatal months and that sleep homeostasis goes through a period of maturation.

A New Study

The authors of the new study, entitled “Development of the Nocturnal Sleep Electroencephalogram In Human Infants,” are Oskar G Jenni, Alexander A Borbély, and Peter Achermann, all of the Institute of Pharmacology and Toxicology, University of Zurich, Zurich, Switzerland. Their findings appear in the Articles in Press section of the Journal of Physiology – Regulatory, Integrative and Comparative Physiology, one of 14 scientific journals published monthly by the American Physiological Society (APS) (www.the-aps.org).

Methodology

Eleven healthy full-term infants (5 boys and 6 girls) were recruited in Zurich of which the data from seven infants were analyzed. All-night home polysomnography was carried out longitudinally at 2 weeks, and at 2, 4, 6, and 9 months after birth. The EEG, submental electromyogram (EMG), electro-oculogram (EOG, differential recording), electrocardiogram (ECG) and respiratory movements were recorded and the signals digitized and transmitted via fiber-optic cable to a computer. The recording equipment was attached in the early evening preceding the usual bedtime and the recordings performed at home in the habitual sleep environment with usual bedtime routines.

Sleep stage scoring was performed at age 2 week and 2 months. Subsequent recordings were scored to take into account that spindles and slow waves are present in older infants, and thus allowing the researchers to differentiate between stage 1 (S1), stage 2 (S2), and slow wave sleep (SWS). Thus, the scoring criteria resembled those used in adults.

A sleep cycle was defined as the succession of a QS/NREMS episode lasting at least 15 min and an AS/REMS episode of at least 5 min duration. Sleep onset AS/REMS was not considered to be the beginning of a sleep cycle, although it was regularly seen in younger infants. Sleep episodes were terminated when > 10 min of indeterminate sleep or wakefulness occurred.

Statistical analysis was performed and age trends of visually scored sleep and cycle variables were assessed. Absolute EEG power density measures were analyzed and significance level was set at p < 0.05. Statistics were based on mean power per QS/NREMS and AS/REMS episode. Results

Highlights of the researchers’ findings include:

Sleep stage and cycle variables derived from visual scoring:
The maximum common length of total recording time (TRT) of 418 min was analyzed. Total sleep time (TST) and sleep efficiency (SE) increased from 2 weeks to 6 months of age, while movement time (MT) and waking after sleep onset (WASO) decreased. However, the decline of the latter did not reach statistical significance. AS/REMS expressed as percentage of TST showed a marked decline during maturation, whereas QS/NREMS increased between 2 weeks and 4 months and stabilized thereafter. The mean duration of the sleep cycle (53-64 min) showed no age-related variation, while the proportion of its two constituent sleep episodes changed in an opposite direction: QS/NREMS episodes increased and AS/REMS episodes decreased.

EEG power spectra of QS/NREMS and AS/REMS:

Spectral power of the sleep EEG showed the typical decline with increasing frequency that is well documented for adult sleep. A striking developmental change in the QS/NREMS spectrum was the emergence of a peak in the frequency range of sleep spindles. The peak appeared at the age of 2 months and was present in each individual spectrum. Visual inspection of the EEG revealed that the spectral peak coincided with emergence of sleep spindles.

Developmental trends in time course of low delta, theta and sigma activity:
In an individual, there are developmental changes in the dynamics of low delta, theta and sigma activity. Power in all three frequency bands among the infants being examined showed a modulation by the QS/NREMS – AS/REMS cycle with low values in AS/REMS and high values in QS/NREMS. Whereas at 2 weeks and 2 months, low delta and theta activity showed a similar time course, a dissociation became apparent at the age of 4 months and thereafter. At 6 and 9 months, theta activity exhibited a declining trend across consecutive sleep episodes. In contrast, low delta activity showed an alternating pattern with high values in every second NREMS episode. At 2 weeks, sleep spindles were not yet present and accordingly sigma activity was at a very low level. From 2 months onwards, when sleep spindles were clearly recognized visually in QS/NREMS, sigma power exhibited high values in QS/NREMS episodes.

Conclusions

The findings of this study suggest that EEG markers of sleep homeostasis appear in the first postnatal months and sleep homeostasis goes through a maturation period. Theta activity and not delta activity seems to reflect the dissipation of sleep propensity during infancy and the nocturnal pattern of specific EEG markers may reflect their different functional roles in the sleep process during infancy.

The American Physiological Society (APS) was founded in 1887 to foster basic and applied science, much of it relating to human health. The Bethesda, MD-based Society has more than 10,000 members and publishes 3,800 articles in its 14 peer-reviewed journals every year.