Sleeping little as a teenager changes how the brain wires itself

That adolescents who sleep less have a differently functioning brain is an observation several years old. What was missing was the direction of the relationship: does short sleep alter brain networks, or do pre-existing brain differences produce short sleep? A study from the psychiatry department of the University of Michigan, published in Developmental Cognitive Neuroscience, tries to answer by bringing together two kinds of data that usually stay apart.

The first is the Abcd Study, the largest American longitudinal research project on brain development in adolescence: 11,878 young people, followed with questionnaires and functional magnetic resonance imaging. The researchers looked for a signature, meaning a recognisable pattern of connectivity between brain networks associated with shorter sleep. They found one, and it held up when applied to an independent subgroup of participants in the same study. Those who cut their sleep hours most over two years show the signature more strongly.

The pattern concerns two lower-order systems above all. The somatomotor network, which governs movement and bodily perception, shows stronger internal connections; the visual network, by contrast, shows weaker ones. These are not the areas one instinctively associates with study or concentration, but they are the systems on which higher functions rest.

The experiment on adults

The second part is what allows talk of direction. Seventy-six adults were scanned after a normal night's sleep and then after laboratory-induced sleep deprivation. Under deprivation the signature found in adolescents appeared more strongly in those same individuals, and the spatial distribution of the two signatures, the young people's and the adults', proved to overlap to a statistically significant degree. Since in the experiment sleep deprivation is the variable the researchers manipulated, the conclusion is that missing sleep produces the connectivity changes rather than the other way round.

Earlier work on the same dataset had already linked a similar pattern to concrete outcomes: lower school grades, lower general cognitive ability, more behavioural problems reported by young people and parents alike. The same analyses show an association with longer screen time, lower household incomes and more disadvantaged neighbourhoods, which makes it hard to isolate sleep from everything that accompanies it.

The study establishes neither how many hours are needed nor whether the damage is reversible by catching up on rest. It says that the relationship between sleep and the brain's functional architecture in adolescence is not merely a statistical coincidence.