Image Credit: Mylene2401
Since Covid, the lack of daily routine while working from home has altered our sleeping patterns, with a lot of us having trouble sleeping altogether. Advice on physical and mental health mentions the importance of rest in staying healthy and happy, but why? Why did humans evolve this physiological need for sleep? What controls our sleep pattern? And what are the effects of sleep deprivation on the body?
Sleeping leaves behind bodies susceptible to predators - yet, the process has persisted in evolution, suggesting it has some function. Sleep certainly has biological importance, as it is conserved in mammals, but many of its functions are still being figured out.
We do, for example, know that sleep plays important roles in metabolism and repairing damaged tissue - yet these could be achieved without the need for loss of consciousness. So, a lot of research focuses on the importance of sleep in the brain, particularly on synaptic plasticity, involved in learning and memory.
The Circadian Rhythm
Sleep can be divided into two states: non-rapid eye movement (NREM) and rapid eye movement (REM), and is controlled by an internal body clock which synchronises physiological processes with the day-night cycle. This ‘master clock’ is thought to be the Suprachiasmatic nucleus (SCN), a region of the hypothalamus in the brain. This region’s importance has been demonstrated in rodent models, where the removal of the rat’s SCN results in the loss of timing of the sleep-wake cycle. Another Nouse article by Isabelle Hall talks about circadian rhythms in detail for those who are interested.
In mammals, the ‘sleep hormone’ melatonin, produced by the pineal gland, encourages the body to sleep. Its secretion is inhibited by light, so it is found in high levels at night and low levels during the day - unsurprisingly, this control of secretion is mediated by the SCN. Interestingly, the blue light from our phones has been found to suppress melatonin secretion similar to how sunlight would during the day. The development of ‘night light’ on Windows and phones aims to battle this, but there is not yet much hard evidence behind the benefits of their use.
Learning & Memory
Trying to cram in a bunch of information the night before an exam? Sleep might be more important than going through that extra lecture. It has been well established that memory formation involves the strengthening of synapses, i.e. the connections between neurons, and involves the switching ‘on’ of plasticity genes, strengthening the synapse. It has been demonstrated that both REM and NREM sleep strengthens previously-established plasticity - in other words, sleep likely helps strengthen established memories. Also, sleep deprivation impairs excitability of neurons, which in turn leads to a more rapid decline of synaptic plasticity than would normally be seen after a normal sleep. In other words, memories are likely to fade more quickly if you’re not getting enough sleep. Interestingly, getting a good night’s sleep the night before learning new information, i.e. before a study day, might make you more likely to retain information. Experiments that led to these findings are discussed in a review by Abel et al., 2013: Sleep, Plasticity and Memory from Molecules to Whole-Brain Networks.
What about dreams? We know they mainly occur during REM sleep, but their function remains largely unclear, as studies often rely on unreliable recounts of dreams after waking. The media has an obsession with the symbolism of dreams, and some even believe in the supernatural nature of dreams - but, you might have a little trouble proving these scientifically. Dreams could serve a more functional purpose, with some studies reporting functional roles of dreaming in processing emotional or traumatic events. Dreams could even serve no purpose at all, being effects of changes in neuronal activity during sleep, first theorised by Harvard psychiatrists Hobson & McCarley.
The importance of sleep can be highlighted in examples of disease and sleep disorders. In Alzheimer’s Disease (AD), as many as 70% of patients also have an associated sleep disturbance, reported by Rongve and colleagues in a 2010 study. Neurodegeneration is accompanied with sleep problems due to loss of an internal body clock, such as by lack of melatonin secretion, but sleep problems themselves can actually exacerbate the neurodegenerative process. For example, some studies have implicated a role of sleep deprivation in a build-up of Amyloid-beta (Aβ), a well-established feature of Alzheimer’s pathology. A review by Spinedi and Cardinali compiles evidence from several of these studies: Neuroendocrine-Metabolic Dysfunction and Sleep Disturbances in Neurodegenerative Disorders: Focus on Alzheimer’s Disease and Melatonin.
Modern Life: Work, Screens, Distractions
In a world obsessed with productivity, many of us are struggling to stay on top of work, spend our leisure time, and get enough sleep without sacrificing one of the three. With the prevalence of technology, and a constant bombardment of distractions, it’s no wonder. For those who have a tendency to be too hard on themselves, remember that it is okay to frequently rest and take naps in between studying. It is smart to prioritise your physical and mental health, especially during these unprecedented times - and actually may be beneficial to your revision. For now, the exact functions of sleep remain largely unclear, but there is undoubtedly a strong relationship between a good night’s sleep and learning and memory, as well as general wellbeing. The importance of sleep certainly cannot be overstated.