Why do we sleep? Nobody really knows, and there are more theories out there than you might think…
Written by: Victor Agrest
Art by: Sia Agarwal
William C. Dement, a noted sleep researcher once joked that “As far as I know, the only reason we need to sleep that is really, really solid is because we get sleepy”.
Sleep has been shown to have some unexplained contribution to survival many times; for example, lab rats that are chronically sleep deprived die after approximately the same length of time that starved rats do, and a well-known hypothesis links sleep with supporting immune system function. The cost of sleep is severe – humans spend approximately a third of their life asleep, during which we obviously cannot eat, drink, reproduce or even defend ourselves. This suggests sleep has some great significance as an essential biological process, or else natural selection would have wiped it from the evolutionary slate long ago.
The philosophy of sleep can be separated into three strands. One set of researchers believe that sleep has a single overarching purpose, deeply rooted in ancient phylogeny. Another set believe that there is no single basis for sleep and that different species, across different stages of life, sleep for varying reasons (to allow recuperative processes to occur vs. energy conservation for instance). And lastly, there are those who categorise the phenomenon of sleep in the same way as the navel, in that it has no meaningful value as an adaptive trait and is merely a byproduct of some yet to be discovered characteristic that does confer an evolutionary advantage. For the former two strands, three broad theories are favoured that attempt to explain why animals sleep.
Sleep has evolved to reduce energy demands
Fluctuations in the availability of food suggests that the ability to reduce energy expenditure is advantageous when food is in low supply. In larger animals, the brain consumes a larger fraction of its total energy than in simpler organisms; sleep is associated with reduced energy expenditure by the brain. However this is too simplistic an argument, and many examples contradict it.
If sleep was truly an energy conserving mechanism it would be comparable to hibernation. However, there are fundamental differences. For example certain animals (ground squirrels, hamsters) post-hibernation, appear to show signs of sleep deprivation. Furthermore, this approach only works for NREM sleep because REM sleep is associated with increased metabolic activity (REM sleep is characterised by high frequency Brain waves).
Ultimately only 80-130 calories are conserved in humans for each night of sleep. While it is possible that energy saving was the main reason for sleep earlier in evolution (REM sleep only found in more complex animals), it is unlikely to be the reason behind the persistence of sleep in mammals.
Sleep has evolved to process information
Higher cortical functions like cognition, attention and memory are known to be impaired by sleep deprivation. Studies have shown that learning and memory performance improve after sleep without repetition of the task. Additionally, imaging studies indicate that during NREM sleep, cortical activity associated with a task learned whilst conscious is depressed, and that during REM sleep such activity is reactivated.
Studies show that during wakefulness there is an unsustainable process of glutamatergic synapse strengthening in the brain – as energy required for maintaining connections and associated firing increases. Afterwards, during sleep there is a synaptic ‘downscaling’ that leaves only those connections that encode the strongest associations intact – reducing energetic and spatial requirements for the maintenance of key circuits.
There are however weaknesses with the theory that sleep has evolved for the consolidation of learning and memory. For example, the theory only relates to those species with a complex brain. As well as this, there is no correlation between the length of sleep and the complexity of the nervous system. And of course, there is the simple fact that learning and memory consolidation are not exclusive to sleep.
Sleep has evolved to allow repair of key cellular components
Different genes are expressed when we are asleep compared to when we are awake. Importantly, many of these differences are associated with major metabolic pathways. They are also associated with the replenishment and trafficking of neurotransmitter vesicles that are used whilst we are awake.
Genetic transcripts related to periods of sleep have been found to encode macromolecules involved in protein synthesis, trafficking and exocytosis. A large number of the genes up-regulated are involved with glutamatergic transmission. Glutamatergic transmission is the major excitatory mode of transmission present in the brain, and is ubiquitous in the animal kingdom. While the strength of this theory (that sleep has evolved to allow repair on a cellular level) is its relevance to all organisms (including unicellular life), this relationship has only been shown so far to be correlative and not causative in nature.
Sleep then, like any other behaviour, is accompanied by a complex set of physiological changes. When comparing mammals with non-mammalian vertebrates and even invertebrates there are few commonalities. On the other hand, there are certain molecular pathways that are widely found in all animals. Establishing the link between the physiological and molecular mechanisms may prove to be key.