There is no doubt that sufficient sleep is needed for good health, and it is well-known that severe sleep deprivation can greatly harm our bodies. Indeed, it is immensely challenging to sustain a state of wakefulness for more than a week as we would imagine, and common physiological changes induced by sleep deprivation include impaired cognition, mood swings and even temporary losses of consciousness.

Why exactly we need to sleep still remains unclear, despite the fact that we spend approximately one-third of our lives doing it. To date, it has long been believed by scientists that sleep is a means by which the brain can “re-balance” itself.

The main idea, which has been the general consensus, is that learning and developmental processes largely take place when we are awake, and this in turn eventually saturates the brain because our neurons get overexcited. Hence, it would follow that sleep has been viewed as a process that is presumably needed to rebalance this state of neuronal overactivity.

Sleeping, waking up and “rebalancing” the brain

Contrary to what was believed for the past decade, recent findings show for the first time that the homeostatic rebalancing of the brain happens when we are awake. In a new study, scientists reported that when the activity of neurons is suppressed in rats, the rebalancing process only occurred specifically when the animals were awake. This implies that homeostatic recovery of neuronal firing is enabled by wake states while simultaneously inhibited by sleep states.

Specifically, the scientists looked at individual neurons in a region of the brain called the visual cortex. The firing rates of neurons was measured in freely behaving rats with obstruction of vision from one eye in both sleep as well as wake conditions. This was achieved by inserting electrodes into the rat visual cortex and tracing the neurons for nine days, which then led to the novel finding that rebalancing of the brain appeared to be more than what was previously postulated.

The logical question, then, is why do our bodies allow the rebalancing act to happen only when we are awake? One proposed reason is that it interferes with critical memory processes when we are asleep, and the sleep-wake boundary could be the body’s clever way of separating key events linked to learning and memory. More importantly, this indicates that certain “waking” processes can complement sleep-dependent ones to provide us with the vitality we need to perform our day-to-day functions, suggesting that investigating the body’s innate mechanism of segregating sleep and wakefulness could be an important direction of research in the future.

Deep sleep and memory consolidation

Our brains remain highly active when we are asleep even though they are not directly coupled to sensory input. More remarkably, our sleep is not homogeneous and can be divided into different phases. Slow-wave sleep (SWS), often referred to as deep sleep, occurs mainly in the first one-third of the night. This has been shown to be important in a type of memory known as declarative memory, or memories which can be recalled such as verbal knowledge and facts. In this regard, it has also been demonstrated in a study that SWS disruption was found to directly lead to a significant increase in sleepiness, with only minimal effects on other daytime functioning aspects. On the other hand, another type of sleep called rapid eye movement (REM) sleep can help with procedural memory, or memories of how to do things such as walking and riding a bike.

In a recent study, scientists visualized brain activity during sleep in the form of electrical signals in different regions of the brain. In particular, they compared the electrical activity in two brain regions, the hippocampus (a small brain region involved in emotion and memory) and the cortex (the outer layer of brain tissue linked to consciousness, learning and memory).

When we are in a state of deep sleep, it turns out that input from the hippocampus reaches the cortex and directly drives the consolidation of recent memories into long-term memories. Amazingly, it was shown that this process involves the activation of specific memories during deep sleep which then leads to a replay of these selected memories, not unlike a recorded video. In addition, changes occur in the synapses of the brain to facilitate long-term memory deposition. What is exciting here is that the computational model proposed by these scientists could subsequently be used to test experimental interventions aimed at enhancing the consolidation of memories.

Sleep and recovery of fatigue – what lies ahead in sleep research?

Even though it is recognized that sleep as a regulated process, there are a variety of factors influencing sleep which make it profoundly difficult to delineate the exact parameters which are being regulated at a specific point in time. Currently, the role of sleep in aiding recovery remains a leading proposition, with a huge body of research correlating sleep with recovery and restoration of behavioral performance and cognitive function in both animals and humans.

Research efforts in the future will be important to bring about further advances in elucidating the cellular processes and molecular pathways which are linked to sleep. At the same time, it is pertinent to realize that sleep is clearly a global and ubiquitous phenomenon, and that manipulation of independent variables at a micro-level alone may not be sufficient to provide a comprehensive understanding of this process, wherein the promise of new and intriguing discoveries beckons.

References

Groeger, J., Stanley, N., Deacon, S., & Dijk, D. (2014). Dissociating Effects of Global SWS Disruption and Healthy Aging on Waking Performance and Daytime Sleepiness SLEEP DOI: 10.5665/sleep.3776

Hengen, K., Torrado Pacheco, A., McGregor, J., Van Hooser, S., & Turrigiano, G. (2016). Neuronal Firing Rate Homeostasis Is Inhibited by Sleep and Promoted by Wake Cell, 165 (1), 180-191 DOI: 10.1016/j.cell.2016.01.046

Mignot, E. (2008). Why We Sleep: The Temporal Organization of Recovery PLoS Biology, 6 (4) DOI: 10.1371/journal.pbio.0060106

Tononi, G., & Cirelli, C. (2014). Sleep and the Price of Plasticity: From Synaptic and Cellular Homeostasis to Memory Consolidation and Integration Neuron, 81 (1), 12-34 DOI: 10.1016/j.neuron.2013.12.025

Wei Y, Krishnan GP, & Bazhenov M (2016). Synaptic Mechanisms of Memory Consolidation during Sleep Slow Oscillations. The Journal of neuroscience : the official journal of the Society for Neuroscience, 36 (15), 4231-47 PMID: 27076422

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