Surprising sleep study sheds new light on the purpose of slumber
New research analyzing data from dozens of sleep studies has identified a dramatic change to the nature and purpose of sleep in babies, occurring around the age of two. The discovery suggests sleep initially aides brain building and neurodevelopment before rapidly switching to a reparative role.
The surprising discovery came through an analysis of more than 60 human and animal sleep studies. Data was gathered tracking total sleep and REM sleep durations, as well as developmental changes to brain and body size. A mathematical model was generated to examine the associations between brain and body development, and sleep phase changes.
The results revealed a remarkably rapid decline in REM sleep in humans at the age of two and a half years – or 2.4 years of age to be exact. Even more surprising, the data was consistent across all species analyzed. Be it pigs, rats or rabbits, when an animal reached its developmental equivalent to a two-and-a-half-year-old human, a sharp drop in the volume of REM sleep was detected.
“I was shocked how huge a change this is over a short period of time, and that this switch occurs when we’re so young,” says Van Savage, co-senior author on the new study. “It’s a transition that is analogous to when water freezes to ice.”
Quantifying this REM sleep reduction, the researchers suggest newborn babies spend around 50 percent of their total sleep time in REM phase but by the age of 50, a person spends as little as 15 percent of slumber in REM sleep.
The study hypothesizes the purpose of sleep in babies is primarily for brain development and neural reorganization. This is not a controversial idea, as the first two years of human life are known to display profound advances in structural brain growth.
We also know that one of the key functions of sleep in adults is to clear the brain of toxic detritus that accumulates throughout a day, and this mechanism primarily takes place during non-REM, slow-wave sleep. What was most surprising about the study was how sharp the age-related transition was from mostly REM to mostly non-REM sleep.
“We identify the specific turning point as occurring at an unexpectedly precise age of around 2.4 years old, reflecting a critical physiological or cerebral developmental change,” the researchers conclude in the study. “In all cases, we see a sharp shift in the scaling of sleep during this period of early development that, to our knowledge, has never been conceptually or quantitatively connected to a shift in sleep function.”
The study raises some compelling questions about the nature of sleep, particularly in terms of early brain development. Further research will need to specifically investigate this correlation between sleep brain wave patterns and regional brain development.
It is unclear at this stage, for example, whether irregular volumes of REM sleep in newborns can be directly linked with abnormal brain development. Plus, the researchers note this early-life sleep phase transition may occur at different times in animals with greater levels of brain development prior to birth.
Either way, co-senior author on the study Gina Poe suggests it is best we, “don’t wake babies up during REM sleep – important work is being done in their brains as they sleep.”
“Sleep is as important as food,” says Poe. “And it’s miraculous how well sleep matches the needs of our nervous system. From jellyfish to birds to whales, everyone sleeps. While we sleep, our brains are not resting.”
The new study was published in the journal Science Advances.