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For scientists, all of the different functions of sleep have not yet been clarified but there is already a consensus that a full night’s sleep helps concentration while the lack of it not only contributes towards low levels of productivity but also brings about long term effects on cardiovascular health and cognitive capacities. These reflect the restorative functions of sleep that renew our capacities to incorporate new information and, simultaneously, enabling the consolidation of the long term memory, adding our daily learning to our accumulated previous experiences. 

Over a century of research finds that sleep is essential to the brain and that our survival functions depend both on sleep and the quality of that sleep1. Nevertheless, the initial theories about sleep did not attribute any active role as regards memories merely thinking that these would receive protection from intrusion by interfering stimuli and thus preventing forgetfulness. Only more recently has there been recognition that memories undergo a process of consolidation that depends on sleep2.

 

According to an article published in Science, in late 2019, memory is a product of cerebral plasticity and its refinement interweaves with the continuous modification of synaptic strengths in which sleeping plays an enormous role3.

 

According to that described, the impulses of cortical activities during the rapid eye movement (REM) phase while sleeping serve to eliminate the weak connections created randomly during the time of watchfulness5. This is when the brain seems to go into a “clean-up mode”, eliminating the weak and undesired memories to make space for storage and become ever more efficient in organising the remainder of the material in the memory. Nevertheless, it is not only the function of forgetting that is today recognised. There is now plentiful evidence in favour of the consolidation of memories function that conveys how the brain structures involved in the doing of tasks are once again activated during the sleep following the training6.

Given that sleep and memory are multidimensional, naturally not all memories benefit in the same way just as sleep itself is not all equal. Studies with humans and animals suggest that the declarative memories are the first to get codified in the hippocampus and in the median surrounding temporal lobe and subsequently transferred gradually to the neocortical zones and with this process associated with deep sleep, also known as slow-wave sleep (SWS). In turn, the non-declarative memories are supported by a cortico-striatal system of base nuclei, which facilitate the acquisition and completion of motor and sequential learning and with this article assuming that the REM phase is associated with their consolidation7,8. However, this relationship between sleep and the consolidation of non-declarative memories remains less clear than that already established for declarative memories and hence subject to further investigation.

 

A century ago, children would sleep more

To facilitate sleep, consistency in the hours of getting up and going to bed has been referenced as fundamental, especially when talking about babies and children. Pediatric associations always warn as to the need to ensure sleep hygiene in order to bring about the good cognitive, behavioural and emotional development of children with the establishment of international recommendations for the number of hours of sleep for the various age groups (See figure below)9. The American National Sleep Foundation furthermore provides a calculator to help in choosing the time of going to bed appropriate to the time one needs to get up at. 

The concern that children do not sleep enough is widespread even if the reality is that children nowadays do sleep less than they would sleep a century ago10. According to American studies, one in every five children and pre-adolescents present symptoms of insonia11,12. The causes identified for this apparent epidemic of sleep-related problems are multiple, however, the changes in life style, in an increasingly digitalised and high-speed world, rank as clear causes for concern among specialists13.

In Portugal, in a study carried out in 2011, the parents of children aged between 4 and 10 years reported their average sleeping time as 9.8 horas, with the vast majority (92.8% of a sample of 574 children) going to bed almost an hour later at the weekend. In addition, children aged between 7 and 10, in responding to a questionnaire, indicated that they generally felt tired (51% of a sample of 310 children)14.

While, on the one hand, the construction of memories receives benefits from sleeping, on the other hand, the deprivation of sleep interferes with cognitive performance that can only be reversed by sleep. The lack of sleep may extend to impacting on the good capacity for judgement, moods, motivations, academic results and even the perception of ongoing events15,16. Sleeping baldly regularly also stands out as a risk factor of relevance for the health and worsening such problems as hypertension, obesity, diabetes, anxiety and aggressive behaviours, among others17,18.

 

To have or not to have a siesta?

However, it is not only sleeping at night that displays associations with the memory. Recent research identifies how, even when students learn prior to a siesta, they remember the information better over the long term. In a study undertaken in Brazil (with the particularity of having taken place in a school environment rather than the usual clinical or laboratory context), evaluated the effects of a siesta following Science and History classes for students aged 10 to 11. The curricular contents of these subjects were measured according to an experimental procedure throughout various weeks. The study concluded that siestas with a duration of between 30 and 60 minutes seem to raise the memory retention of the curricular content by around 10%19, hence, underpinning the need for reflection on the benefits of siestas after classes in order to boost the consolidation of the material taught in the class.

There is still a great deal to study about the simple act of sleeping but what is certain is that the brain does not accept any apologies and sleeping, to an extent adjusted by age, has to be a priority for all students. Their health and memory shall both thank them.


References

1 Peigneux, P., Schmitz, R., Urbain, C., «Sleep and forgetting», em S. Della Sala (Ed.), Forgetting, Hove, Psychology Press, 2010, pp. 165-184.

2 Rasch, B., & Born, J., «About sleep’s role in memory», Physiological Reviews, 93(2), 2013, pp. 681-766.

3 Cirelli, C., & Tononi, G., «Linking the need to sleep with synaptic function», Science, 366 (6462), 2019, pp. 189-190.

4 Stickgold, R., «Sleep-dependent memory consolidation», Nature, 437(7063), 2005, pp. 1272-1278.

5 Crick, F., & Mitchison, G., «The function of dream sleep», Nature, 304(5922), 1983, pp. 111-114.

6 Peigneux, P., Laureys, S., Fuchs, S., Collette, F., Perrin, F., Reggers, J., et al., «Are spatial memories strengthened in the human hippocampus during slow wave sleep?», Neuron, 44(3), 2004, pp. 535-545.

7 Frankland, P. W., & Bontempi, B., «The organization of recent and remote memories», Nature Reviews Neuroscience, 6(2), 2005, pp. 119-130.

8 Ackermann, S., & Rasch, B., «Differential effects of Non-REM and REM sleep on memory consolidation?», Current Neurology and Neuroscience Reports, 14(2), 430, 2014, pp.1-10.

9 Hirshkowitz, M., Whiton, K., Albert, S. M., Alessi, C., Bruni, O., DonCarlos, L., … Adams Hillard, P. J., «National Sleep Foundation’s sleep time duration recommendations: methodology and results summary», Sleep Health, 1(1), 2015, pp. 40-43.

10 Matricciani, L. A., Olds, T. S., Blunden, S., Rigney, G., Williams, M. T., «Never enough sleep: a brief history of sleep recommendations for children», Pediatrics, 129(3), 2012, pp. 548-556.

11 Calhoun, S. L., Fernandez-Mendoza, J., Vgontzas, A. N., Liao, D., & Bixler, E. O., «Prevalence of insomnia symptoms in a general population sample of young children and preadolescents: gender effects», Sleep Medicine, 15(1), 2014, pp. 91-95.

12 Simola, P., Laitalainen, E., Liukkonen, K., Virkkula, P., Kirjavainen, T., Pitkäranta, A., & Aronen, E. T., «Sleep disturbances in a community sample from preschool to school age», Child: Care, Health and Development, 38(4), 2011, pp. 572-580.

13 LeBourgeois, M. K., Hale, L., Chang, A.-M., Akacem, L. D., Montgomery-Downs, H. E., & Buxton, O. M., «Digital Media and Sleep in Childhood and Adolescence», Pediatrics, 140(Supplement 2), 2017, S92–S96.

14 Loureiro, H., Pinto, H. R., & Paiva, T., «Adaptation and validation of children’s sleep habits questionnaire and sleep self-report for portuguese children – preliminary results», Sleep Medicine, 12, 2011, S88.

15 Beebe, D., «Cognitive, behavioral, and functional consequences of inadequate sleep in children and adolescents», Pediatric Clinics of North America, 58/3, 2011, pp. 649-665.

16 Paiva, T., & Rebelo-Pinto, H., «Clínica do sono da criança e do adolescente», em T. Paiva, M. Andersen, S. Tufik (Eds.), O Sono e a Medicina do Sono, São Paulo, Ed. Manole Ltda, 2014, pp. 599-624.

17 Orzeł-Gryglewska, J., «Consequences of sleep deprivation», International Journal of Occupational Medicine and Environmental Health, 23(1), 2010, pp. 95-114.

18 Chen, X., Beydoun, M. A., & Wang, Y., «Is Sleep Duration Associated With Childhood Obesity? A Systematic Review and Meta-analysis», Obesity, 16(2), 2008, pp. 265-274.

19 Cabral, T., Mota, N. B., Fraga, L., Copelli, M., McDaniel, M. A., & Ribeiro, S., «Post-class naps boost declarative learning in a naturalistic school setting», Npj Science of Learning, 3(1), 14, 2018, pp. 1-4.


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