Development: Effect of sleep on the human brain ‘suddenly changes’ in childhood, study shows 

As children grow the effect that sleep has on the brain changes from supporting memory and learning to maintenance and repair, a study has found.

US experts found that the shift in function occurs at around the age of two-and-a-half — a time when several big brain transformation were previously known to occur. 

Most animals need sleep to repair damage caused by stress and to recognise neural patterns, which are key to boosting learning and memory skills. 

‘The pervasiveness of sleep during development and throughout the animal kingdom suggests that it is a biological process that is necessary for survival,’ said paper author Junyu Cao of the University of Texas at Austin.

‘Although we spend approximately a third of our life asleep, its explicit physiological and evolutionary function remains unclear, with a myriad of hypotheses.’

In their study, Professor Cao and colleagues analysed sleep datasets — taken from children aged between 0–15 — using models that focussed on each child’s brain’s metabolic rate, volume and the amount of time spent in REM sleep.

REM — or ‘rapid eye movement’ — is one of the five stages of sleep that occurs several times a night and is when dreaming takes place.

‘We created a novel mechanistic framework for understanding and predicting how sleep changes,’ explained Professor Cao.

‘Because data are seldom analysed in a way that connects them with mathematical models or quantitative predictions, conclusions about the function of sleep have remained slow to evolve,’ she added.

Neural reorganisation, an important part of learning and memory, takes place before children reach the age of two-and-a-half years old, the researchers found.

After this point, the brain appears to stop reorganising, and focuses instead on protecting and repairing neural networks.

According to the team, the change in sleep function was not gradual, but rather like ‘water turning to ice’. 

‘Our findings reveal an abrupt transition, between two and three years of age in humans,’ said paper author Van Savage of the University of California, Los Angeles.

‘Specifically, our results show that differences in sleep across animal groups and during late ontogeny (after two or three years, in humans) are primarily due to sleep functioning for repair or clearance.’

‘Changes in sleep during early ontogeny (before two or three years) primarily support neural reorganisation and learning.’

Neuroplastic reorganisation, the brain’s ability to learn by changing its structure and function, took place during REM rather than non-REM sleep phases, the researchers also found.

The researchers hope to examine the sleep function change in animals with shorter development periods, where it occurs earlier, and can even take place before birth.

The full findings of the study were published in the journal  Science Advances.