High Sensory Sensitivity in Infants Linked to Shallower Deep Sleep

Parents of sensory-sensitive babies often describe a child who wakes at the slightest sound, who takes longer to settle, who seems perpetually alert even when visibly tired. A study published in the journal Sleep now adds neurological detail to that familiar picture: in infants aged 8 to 11 months, high sensory sensitivity correlates with reduced depth of slow-wave sleep - the restorative deep sleep stage critical for brain development and memory consolidation - and this difference appears even when the room is quiet.

The research, from the University of East Anglia's School of Psychology, used electroencephalography (EEG) to record brain activity directly during naps, allowing the team to measure not just whether babies were in deep sleep but how deep that sleep actually was. The critical metric was the amplitude of slow waves - the large, synchronized brain oscillations that define deep sleep. Smaller, weaker slow waves indicate shallower deep sleep even when the baby is technically in the right sleep stage by other measures.

The Experimental Setup

Each of the 41 babies who contributed data to the final analysis visited UEA's sleep laboratory and completed two naps under different conditions. One nap took place in a quiet room; the other in a room where gentle beeps - calibrated to the volume of normal conversation - sounded every few seconds. This comparison was designed to isolate the effect of ambient sound on sleep quality, while also allowing the researchers to observe baseline sleep characteristics independent of external noise.

Parents completed questionnaires assessing their infant's sensory sensitivity and typical behaviors before the sleep sessions. This provided the sensory profile measure that the team then related to the EEG recordings. The study enrolled infants both with and without older autistic siblings - not because those infants can be identified as autistic at 8 to 11 months, which is not possible, but because having an autistic sibling substantially increases the statistical probability of also developing autism, and because sensory sensitivity traits, common in autism, are often measurable in infancy before any diagnosis is possible.

What the Brain Waves Showed

The findings confirmed that noisy conditions disrupted deep sleep for all infants - that part was expected. The more informative result was what happened in the quiet condition. Even without external sound to contend with, infants rated higher on sensory sensitivity produced slow waves with lower amplitude than their less sensitive peers. They spent similar amounts of time in deep sleep; the sleep was simply shallower in a measurable neurological sense.

Prof. Teodora Gliga, who led the study, described the distinction as important: the issue is not that sensitive infants fail to enter deep sleep, but that the depth and quality of that sleep - as reflected in the electrical activity of their brains - is reduced. This matters because the restorative and developmental functions of deep sleep are tied to slow-wave activity itself, not merely to clock time spent in the sleep stage.

The noise-exposed condition showed an amplified version of this pattern. Infants with high sensory sensitivity experienced considerably more disruption to their deep sleep when the beeps were playing, showing a larger performance gap relative to less sensitive infants than was observed in the quiet condition. Both environmental sensitivity and individual sensory wiring appear to contribute independently to the effect.

What This Study Cannot Conclude

The sample of 41 infants is small, which limits the statistical power to detect subtle effects and means individual variation has an outsized influence on the results. The study design does not follow these children over time, so the long-term consequences of shallower deep sleep in infancy - for development, for behavior, for later sleep architecture - cannot be determined from this data alone.

The use of older autistic siblings as a proxy for autism likelihood is methodologically sound but introduces complexity in interpretation. Not all infants with autistic siblings will develop autism, and not all will share their sibling's sensory profile. The study is examining sensory sensitivity as a continuous trait, not autism as a category, and the findings describe that trait's relationship to sleep - a valid question independent of any eventual diagnosis.

The researchers are also careful not to claim that shallower sleep causes autism or is caused by it; the study identifies a correlation between sensory sensitivity and sleep depth at a specific developmental window, without establishing mechanism or directionality.

Implications for Families

Prof. Gliga noted that reducing nighttime noise may help particularly sensitive babies, but the quiet-room data make clear it is not sufficient on its own. Infants with high sensory sensitivity show shallower slow-wave sleep regardless of environmental noise, suggesting that the solution - if there is one - likely involves supporting the brain's own ability to filter sensory input during sleep, rather than simply managing the external environment.

The study opens questions about whether interventions targeting sleep quality in sensory-sensitive infants could have developmental consequences worth pursuing, but those questions require much larger and longer-term studies to answer. The funded next step, according to the authors, is further research into how deep sleep might be actively supported in these infants - a more complex challenge than simply turning off the lights and lowering the volume.

The research was funded by the Wellcome Trust. Collaborators included Prof. Rachael Bedford at Queen Mary University of London and Dr. Alpar Lazar at UEA.