Why do we need so much sleep and babies? Researchers are now providing new clues as to the importance of the still mysterious state of rest. Their analysis of extensive sleep data shows that around two and a half years of age, the primary role of sleep shifts from building nerve connections to maintaining and repairing the brain quite abruptly. This shift in function appears to exist in other mammals at similar developmental stages, the scientists say.
You can’t do without it: Sleep deprivation can lead to life-threatening conditions and chronic lack of sleep makes people and animals sick, as many studies have shown. Despite the apparent importance of the state of rest, which takes up about a third of our lives, there is still a lot of confusion about the exact functions of sleep and why its characteristics change over the course of life. One basic principle, however, is clear: During slumber, only the body takes a break – but not the brain.
Many special meanings are ascribed to this restlessness in the head, but in principle there are two main functions that take place during sleep: The stress damage in the nervous tissue that occurs while awake is repaired and waste substances are disposed of. In addition, neuronal reorganization processes take place: New nerve connections are established, which are important for learning and other cognitive skills, partly on the basis of experiences in the waking state. The relative importance of these two assumed basic functions, however, seems unclear so far. The changes in sleep behavior during childhood, however, already suggested that the focus of brain functions shifted during development.
Questioning look at the restlessness in the head
An international team of researchers from the fields of neuroscience, biology, mathematics and statistics has now followed this trail. As part of their study, they carried out the most comprehensive statistical analysis of sleep to date. They evaluated data from more than 60 sleep studies with humans and some mammals. The data included information about the characteristics of sleep during development – such as sleep data from children ages 0-15. On the basis of the measured variables on brain metabolism rates, brain volume and the length of time spent in the various phases of sleep, the researchers developed models of sleep functions and their changes in the course of development.
As they report, their results provided further evidence of the role of the so-called REM sleep phase, in which the eyes move, and the NREM sleep phase, in which this activity does not occur. According to the researchers, it emerges from the data that REM sleep is primarily responsible for the reorganization of the brain and learning processes that dominate in early development. During NREM sleep, on the other hand, the repair and maintenance functions of the nerve tissue take place.
From when the maintenance work predominates
In the study results, a strong and abrupt shift in this connection is now clear, the scientists report: At around 2.4 years of age, the dominance changes from REM sleep to NREM sleep – accordingly, there is a shift in the meaning of sleep from neural sleep Reorganization for maintenance. Presumably, the decrease in demand is also connected with this: People have to sleep less because not so many new nerve connections have to be formed.
The results of the study also show how the proportion of REM sleep decreases with the increase in brain size during development. While newborns spend about 50 percent of their sleep time in REM sleep, this percentage drops to about 25 percent by the age of ten and then continues to decrease with increasing age. Adults older than 50 years only spend about 15 percent of their sleep time in REM sleep.
The clear tipping point in the shift between sleep phases at around two and a half years apparently marks the main change in the development of sleep, say the researchers. “I was surprised how clear this change is in a short period of time and that this change occurs when we are this young,” says co-author Van Savage of the University of California in Los Angeles.
As his colleague Gina Poe points out, the results also clarify a rule: “Babies should not be woken up during REM sleep because important work is going on in their brains during this phase,” says Poe.
The scientists now see in their results important information for sleep research and starting points for further investigations. Their preliminary results on the analysis of sleep data from rabbits, rats and guinea pigs suggest that a transition in sleep function similar to that in humans also occurs during their development. The researchers now want to further explore to what extent this could be a general principle. “I am very interested in whether our results can also be extended to other mammals,” says lead author Junyu Cao from the University of Texas at Austin.
Source: University of California – Los Angeles, Santa Fe Institute, Articles: Science Advances, doi: 10.1126 / sciadv.aba0398