Text from Valeria Jaramillo
A recent study aimed to unravel the big question of why we need to sleep. Excitingly, the reason for why we sleep appears to change abruptly at the age of 2 to 3 years.
Sleep is a universal phenomenon that is present in all animal species studied so far 1. Thus, scientists agree that sleep serves a vital restorative function. However, the underlying mechanism for restoration – what exactly happens in brain and body in the nightly recovery period – remains a stimulating debate among scientists. Furthermore, the two main components of sleep – REM (rapid eye movement) and NREM (non-rapid eye movement) sleep – seem to enable different restoration functions. The proportion of REM sleep is intriguingly high in infants and decreases with older age 2. The new examination of Prof. Van M. Savage and his research group at the University of California in Los Angeles tests whether the core function of sleep changes across development. To investigate this question, they conducted a mathematical modeling study (published in Science Advances) 3.
In a first step, the team built mathematical models for the two most popular hypotheses of sleep function, based on observations from different species and across development. The first hypothesis claims that sleep prevents damage of brain cells (neurons) by allowing these to rest and by removing toxic waste products accumulated during a period of activity 4,5. The second hypothesis postulates that sleep reorganizes brain connectivity which is crucial for powerful brain function and learning 6,7. In a second step, these hypotheses were tested using experimental data across development. Existing studies were combined in a meta-analysis, and total sleep time, brain energy expenditure, brain size, the contacts between neurons, and the proportion of REM within total sleep were examined.
Findings were intriguing and suggest that the core function of sleep changes in the course of development 3: During early development sleep predominantly promotes brain reorganization whereas during late development the neuronal repair function of sleep is more dominant. Excitingly, the transition from a predominantly brain reorganization to a neuronal repair function occurred abruptly at the age of approximately 2.4 years. Finally, brain reorganization seemed to be enabled in REM but not in NREM sleep.
Taken together, this study elegantly indicates that sleep – and REM sleep in particular – enables a time for shaping brain circuits in infants. When we then grow older (and possibly become parents), sleep becomes more important for permitting repair and toxic waste removal, thereby preventing brain damage.
1. Cirelli C, Tononi G. Is Sleep Essential? [Internet]. PLoS Biol. 2008;6(8). DOI: 10.1371/journal.pbio.0060216
2. Roffwarg HP, Muzio JN, Dement WC. Ontogenetic development of the human sleep-dream cycle. Science 1966;152(3722):604–619. DOI: 10.1126/science.152.3722.604
3. Cao J, Herman AB, West GB, et al. Unraveling why we sleep: Quantitative analysis reveals abrupt transition from neural reorganization to repair in early development [Internet]. Sci. Adv. 2020;6(38):eaba0398.
4. Vyazovskiy V V, Harris KD. Sleep and the single neuron: the role of global slow oscillations in individual cell rest. Nat. Rev. Neurosci. 2013;14(6):443–451. DOI: https://doi.org/10.1038/nrn3494
5. Jessen NA, Munk ASF, Lundgaard I, Nedergaard M. The Glymphatic System: A Beginner’s Guide. Neurochem. Res. 2015;40(12):2583–2599. DOI: 10.1007/s11064-015-1581-6
6. Tononi G, Cirelli C. Sleep and the price of plasticity: from synaptic and cellular homeostasis to memory consolidation and integration. Neuron 2014;81(1):12–34. DOI: 10.1016/j.neuron.2013.12.025
7. Rasch B, Born J. About Sleep’s Role in Memory . Physiol. Rev. 2013;93(2):681–766. DOI: 10.1152/physrev.00032.2012