Of Sleep and Sleeplessness: One More Reason to Get a Good Night’s Sleep.
By: Beatrice Ballarin
Magical things happen at night: while lying still for hours we slip into time travelling, shape shifting, the transposing of familiar faces from our childhood into modern spaces and time. Nonetheless, we sleep through it all every night, if we’re lucky. It was just a dream: an involvement of the sleeping brain into an emotional animation. We don’t know why we sleep but we know it has to be done as part of our daily routine. Even in the epic poem, Odyssey, Homer declared “There is a time for many words, and there is also time for sleep.”
Why do we sleep anyways? Scientists don’t have a satisfying answer to this question.
Sleep is a complex and mysterious phenomenon, or at least it is to many people. Assuming that we’ll be living a long and (hopefully) happy life until we are 90 years old, we will have spent a total of 32 years sleeping.
Aristotle, the father of logic, was the first to attempt an answer to this question in his work, On sleep and sleeplessness, from 350 B.C.. He considered sleeping merely a passive state, defined as an unremarkable and unimportant period that lacked the ‘sense of perceptions,’ the lack of awareness into the world. Surprisingly, in this current time, our perception of sleep has not changed that much from Aristotle’s view. We seem to have forgotten the importance of a good night’s sleep. Is sleep overrated? When the hours of the day are not enough to conclude our work, staying up late seems to be a good solution; as a result, we deprive ourselves of adequate sleep. Why do we sleep, anyways? It seems like such an unproductive use of time. Couldn’t we just stay awake and do some work?
Nevertheless, some hours every night have to be dedicated to the act of sleeping. While our perception of sleep has not changed much since Aristotle’s time, our understanding of its functions has evolved.
Sleep likely has multiple functions. An increasing area of interest in neuroscience is the link between sleep and memory, as anyone who has ever had a sleepless night knows it can ruin the capacity to retrieve information. From a synaptic point of view, sleep allows synaptic consolidation and strengthening of the connections among neurons, making it critical to plastic remodeling to occur within the neural circuits.1 If it is true that sleep results in the processing of information acquired while awake, then in theory it should be possible to detect some sort of neuronal firing during sleep. An intriguing study published in Science attempted to address this question by recording from single neurons in songbirds.2 Yes, songbirds. By recording neurons in the forebrain region that is involved in the production of the song, the group, led by Dr. Margoliash, was able to show that the timing and structure of the firing pattern elicited by the playback of the song during sleep matched the activity during the daytime singing. It was as if the song-producing circuits were unconsciously and spontaneously rehearsing these patterns while sleeping.
Historically, REM (rapid eye movement) sleep, the stage of sleep where we dream the most, has been linked to spatial and emotional memory consolidation. However, whether REM sleep plays a direct role in memory consolidation remains controversial. The influence of REM sleep on memory has been hard to study because of the transient nature of REM sleep and the devastating effect of sleep deprivation experiments. However, Richard Boyce and his colleagues at McGill University have recently taken a novel approach to establishing this link.3 Using optogenetics in mice, they were able to interrupt REM sleep by reducing theta wave oscillation in the cortex and hippocampus (the main memory structure in the brain) during REM sleep without disturbing sleeping behavior. When they looked into the animal’s spatial memory ability, those with undisrupted REM sleep spent more time exploring an object moved to a novel location than an unmoved object, while mice with disrupted REM sleep seemed to not remember the objects’ earlier positions. And that’s not it. Surprisingly, the mice with disrupted REM sleep lose their natural preservation instinct, showing fewer signs of fear in a place where they had previously received electric shocks during a fear conditioning experiment. Interfering with theta waves during other stages of sleep didn’t cause the same memory dysfunction, conferring to REM sleep this unique role of memory consolidation. Magical things happen at night, especially during REM sleep.
Another innovative concept that attempts to explain why we sleep is based on the “aggregation theory”.4 Let’s take a step back: it is known that one of the main pathologies that characterizes Alzheimer’s disease (AD), one of the most pervasive and debilitating forms of dementia, is the aggregation of amyloid-β (Aβ) in the extracellular space, predominantly in the cortex. It is also known that clinical symptoms present 10 to 15 years later and by that time, there is already a consistent neuronal and synaptic loss.
An interesting set of experiments conducted by Holtzman and his colleagues at Washington University have shown that in a mouse model of AD the sleep-wake cycle was disrupted once Aß plaques formed.5 They were able to rescue the normal sleep pattern by eliminating the plaques, suggesting that plaque formation was somewhat responsible for the sleep disturbances. Could this suggest that sleep disruption might be a risk factor for developing Aβ deposition and possibly AD? Can sleep disturbance be an early marker for brain changes? One thing is for sure: Aβ aggregation may cause sleep disturbances long before the clinically-detected AD symptoms.6 Early detection of these sleep disturbances could potentially delay the onset of the symptoms of dementia.6 This study builds on previous work regarding memory consolidation, leading toward a new understanding of why we sleep: to clear potentially toxic proteins from the brain.
Although these studies only address a few aspects of sleep, scientists have made tremendous strides in discovering what happens while we sleep and what mechanisms are involved. This is definitely not the end of this chapter, leaving space for generating new knowledge about this essential part of life that we spend silent, still, and dreaming. In Italy, we have a saying: “La notte porta consiglio” (“the night will carry advice”). So let’s sleep on this one.
- Diekelmann, S. & Born, J. The memory function of sleep. Nat. Rev. Neurosci. 11, 114–126 (2010).
- Dave, A. S. & Margoliash, D. Song replay during sleep and computational rules for sensorimotor vocal learning. Science 290, 812–816 (2000).
- Boyce, R., Glasgow, S. D., Williams, S. & Adamantidis, A. Causal evidence for the role of REM sleep theta rhythm in contextual memory consolidation. Science 352, 812–816 (2016).
- Lucey, B. P. & Holtzman, D. M. How amyloid, sleep and memory connect. Nat. Neurosci. 18, 933–934 (2015).
- Kang, J.-E. et al. Amyloid-beta dynamics are regulated by orexin and the sleep-wake cycle. Science 326, 1005–1007 (2009).
- Mander, B. A. et al. β-amyloid disrupts human NREM slow waves and related hippocampus-dependent memory consolidation. Nat. Neurosci. 18, 1051–1057 (2015).