Infant limb movement during sleep follows a rhythmic cycle

Infants' activity whilst sleeping is rhythmic, according to a new study from the University of Surrey. Findings provide crucial insight into sleep cycles of infants in the first year of life.

In the largest study of its kind, scientists from Surrey, led by Dr Eva Winnebeck, investigated the sleep cycle development of 152 infants at age 3, 6 and 12 months.

To learn more, movement levels of infants were closely monitored using an actigraphy device (a non-invasive, wearable piece of technology placed on the ankle of infants) for 10 days at each age. Parents were also provided with an actigraphy device for their wrist to measure their movement whilst sleeping.

During the first year of life, sleep undergoes a remarkable development. From multiple short sleep bouts scattered almost evenly across the 24-hour day, an infant's sleep gradually moves into the nighttime and consolidates into longer bouts, much to the relief of their parents.

Infant sleep, despite its importance for neurological and physical development, is an under-researched area of science. To learn more, we have gone back to basics and are examining the movement of infants whilst they sleep. This was the foundation of sleep research prior to the discovery of REM and non-REM sleep and provides a useful window into infant sleep outside the sleep lab. The more we learn, the more we can pinpoint healthy from unhealthy sleep and help parents and doctors to catch problems early to support infants in their optimal development."

Dr. Eva Winnebeck, Lecturer in Chronobiology, University of Surrey

Using signal processing techniques in combination with the analysis of more than 35,000 hours of sleep data from the infants, scientists identified the existence of rhythmic patterns of inactivity in infants from three months of age. Cycles of inactivity typically lasted 60 minutes and increased by 10 minutes until 12 months. Unsurprisingly, cycles were shorter in infants than their parents whose cycles of inactivity lasted 81 minutes on average. These findings are in line with smaller studies examining REM and non-REM sleep cycles and confirm that sleep cycles gradually lengthen over development

Limb inactivity was also found to be higher at the start of the sleep bout and did decrease as time progressed, again mirroring REM and non-REM dynamics.

Scientists also found that infants who were still breastfed at 12 months had longer cycles of inactivity compared to non-breastfed infants. Mothers who were still breastfeeding their infant at this age were also found to have a longer cycle length, by approximately 6.7 minutes.

Scientists hypothesise that breast feeding, due to its hormonal content (higher cortisol during daytime and higher melatonin at night) could potentially lead to circadian entrainment in infants and contribute to the maturation of the circadian system. However more research is needed in this area.

Dr Grégory Hammad, a Visiting Researcher at the University of Surrey and the first author of the study, said:

"It seems obvious that inactivity levels correlate with the onset of sleep as when infants are asleep, they are less likely to move. However, what we have found is that inactivity of limbs is in fact itself rhythmic during sleep and follows a pattern similar to the cyclic alternation of non-REM and REM sleep across the night. The cycles of inactivity increased in length with a child's age and are a good indicator of what sleep cycle an infant is in. Our finding will help advance our understanding of the intricate relationship between sleep and development."

This study was published in the journal: SLEEP