Why Do We Sleep? The Science Behind Our Need for Rest

Everyone needs sleep, but it’s exact purpose has been a point of research with no conclusive answers for a long time. A good night’s sleep dictates how well the brain cells communicate with each other to allow for effective brain function. It is as essential to our well-being as food and water. It helps in creating and consolidating new memories, remove toxins from the brain, and even assist in metabolism, immune function, and body temperature regulation. A lack of sleep has serious deleterious effects and increased the risk of a whole range of illnesses including Alzheimer’s, cardiovascular diseases, and depression. It also leads to sleep deficits that accumulate and can take a long time to return to baseline.

The Science of Sleep

Sleep can be divided into 2 stages; Rapid Eye Movement or REM sleep and Non-Rapid Eye Movement or NREM sleep. NREM is further divided into 4 stages with stages 3 and 4 categorised as Slow Wave Sleep or SWS. Sleep functions in cycles of NREM and REM sleep and the length of each cycle is about 90 minutes. Not all stages of sleep occur for the same time period and each has distinct brain activation or deactivation features. Through the progression of NREM sleep, brain activity as measured through EEG shows considerable slowing down and SWS is characterised by very low frequency waves (Less than 1 Hz). Contrary to this, REM sleep is characterised by high frequency waves (30-80 Hz) with recurrent outbursts of rapid eye movement accompanied by muscle atonia and an elevated arousal threshold. The high frequency activity in REM is similar in pattern to the wake-state and thus it is also sometimes referred to as ‘paradoxical sleep’.

Role of sleep in memory consolidation

One theory about the function of sleep has to do with memory consolidation. While acquisition of memory happens during wakefulness, consolidation is facilitated by sleep. Sleep functions to not only restore lost memories but also enhance supplementary learning. The system consolidation theory states that the hippocampus and neocortex interaction is what leads to the stabilization of newly acquired memories. Research has shown that performance improvements continue for up to 24 hours after active learning is over and a restful night leads to greater speed and accuracy when recalling this information the next day. These gains are, however, lost if there is disruption to REM sleep. Hippocampal-neocortical interactions are activated repeatedly during consolidation and recall which strengthen the flow of information from the hippocampus to neocortical structures. Sleep is said to play a role in the reactivation of these interactions. Slow wave activity assists hippocampal activation and sleep deprivation was found to be directly linked to poor performance in memory tasks.

Role of sleep in development and maturation

Human babies express an extremely high need for sleep, especially REM sleep at an age that exhibits heightened plasticity of the central nervous system (CNS). This need for REM sleep declines as they age and finally plateaus at maturity suggesting that it has a role to play in brain development. Some studies suggest that this is because sleep helps in development of the visual system, along with sensorimotor development.

Role of sleep in energy conservation

It might seem almost obvious that sleep helps in energy conservation. It was back in the 1970s that this idea was first suggested by psychologists. It is possible that sleep evolved to make up for the loss of energy during wakefulness due to the high cost of endothermy, the process by which animals generate heat to maintain their body temperature. Sleep drops the body temperature by a significant amount thereby conserving energy. This could also be connected to why new-borns need so much sleep!

Role of sleep in neural waste disposal

The lymphatic system that is responsible for clearing out waste from the body functions in a slightly different way in the brain. The brain features a cerebrospinal fluid (CSF) that circulates through the brain removing interstitial proteins like β amyloids when it comes in contact with interstitial fluid (ISF). Studies have shown that during wakefulness, the interstitial space is greatly reduced causing a resistance to the movement of CSF. A build-up of β amyloid is directly linked to loss of memory transformation and retention abilities especially seen in older generations and this has been found to be dependent on the quality, or lack thereof, of Slow Wave Activity (SWA) and NREM sleep. They suggest that treating abnormalities in SWS could help combat diseases such as Alzheimer’s as good quality sleep has been shown to reduce cognitive decline and increase resilience to cognitive impairment.

Take-Aways

1.     Sleep deficits accumulate! If you don’t get enough sleep, you are going to have to make up for it at some point. Research shows that it can take up to 9 days to make up for even 2-3 hours of lost sleep.

2.     Sleep can be divided into Rapid Eye Movement (REM) and Non Rapid Eye Movement (NREM) sleep. Both serve specific functions.

3.     Sleep helps consolidate newly acquired memories by activating the hippocampus. Sleep deprivation is shown to lead to poor memory consolidation.

4.     Antidepressants can have an effect on the quality of sleep, however, these can be tackled efficiently by modifying the timing and dosage of medication, as well as combining it with talking therapies.

5.     Sleep helps in the development and maturation of the brain, as well as in conserving energy in all endothermic animals. Perhaps this also explains why babies sleep so much!

6.     Sleep helps remove the build-up of interstitial protein in the brain that has shown to be directly linked to memory related diseases like Alzheimer’s.

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