I propose that the need for sleep and the occurrence of dreams are intimately linked to the physical processes underlying the continuing replacement of cells and renewal of biomolecules during the lives of higher organisms. Since one major function of our brains is to enable us to react to attack, there must be truly compelling reasons for us to take them off-line for extended periods, as during sleep. I suggest that most replacements occur during sleep. Dreams are suggested as part of the means by which the new neural circuitry (I use the word in an informal sense) is checked.
Deep Dive into Why dream? A Conjecture on Dreaming.
I propose that the need for sleep and the occurrence of dreams are intimately linked to the physical processes underlying the continuing replacement of cells and renewal of biomolecules during the lives of higher organisms. Since one major function of our brains is to enable us to react to attack, there must be truly compelling reasons for us to take them off-line for extended periods, as during sleep. I suggest that most replacements occur during sleep. Dreams are suggested as part of the means by which the new neural circuitry (I use the word in an informal sense) is checked.
Sleep is a remarkably universal phenomenon among higher organisms. Indeed, many of them would die without sleep. Whereas hibernation, a distinct phenomenon, is readily understood in terms of energy balance, the reasons for sleep and for associated dreaming are by no means fully established. Indeed, some of the standard ideas are arguably implausible [1]. This present note describes a conjecture as to the role of dreaming. The ideas, in turn, point to a further conjecture about the need for sleep.
I have tried to avoid any controversial assumptions. All I assume about the brain is that it comprises a complex and (at least partly) integrated network of components, and has several functions, including the obvious ones of control of motion, analysis of signals from sensors, computing (in the broadest sense), memory, and possibly other functions. Some brain components may provide a degree of redundancy; others may have more subtle functions not yet defined. I do not try to identify detailed physical mechanisms. In particular, my conjectures do not need an understanding of the means by which information from DNA or elsewhere mediates or controls brain structures or their replication.
Background .
Most cells in the body are replaced every few months. This seems the case for stem cells [2] and for fat cells [3]. Neurogenesis, the creation of new neurons, is established for neurons that migrate to the olfactory bulb and also for those in the dentate gyrus of the hippocampus associated with spatial memory [4]. There are exceptions, e.g., some eye structures and some iron-containing molecules. Proteins within neurons are replaced with half-lives from hours to weeks, but (private communication, Luca Turin) many, perhaps most, neurons are never replaced throughout life. Likewise, most biomolecules are replaced on similar time scales. Thus basic molecular constituents of some components of the brain -memory and networks of whatever nature -are largely replaced every few months. As regards memory, irrespective of whether it is maintained physically in molecular or cell-level structures, there has to be regular, perhaps continuous, refreshment of some of the relevant molecular hardware. Related to these ideas of physical replacement, I note two important observations. First, there is evidence that those nerve connections that have built up during the working day are cut back to some degree during sleep, partly to edit information and partly to reduce energy use [5]. Secondly, during sleep, certain categories of genes are enhanced. These prove to be those that are involved in rebuilding the molecules that are essential for cell function. Thus the enhancement affects genes for all stages of cholesterol synthesis. Cholesterol has a role in signalling, since it is a major component of the lipid rafts in cellular membranes that bring together signalling molecules such as neurotransmitters and receptors, and so affect the efficiency of signalling [6]. This is consistent with conjecture A below that much of the regular rebuilding of the neural system occurs primarily during sleep.
The renewing brain surely makes use of any spare capacity and the redundancies. Spare capacity seems essential to the human brain. Not surprisingly, the brain has evolved to have robust neural circuitry, rather than maximally efficient circuitry: lean management (that fashion of managers in industry) is not an option. The relevant parts of the brain for present purposes are presumably those showing neural correlates of consciousness, of which there are quite a few [7]. Lesion studies suggest there is not much redundancy evident in the hindbrain or midbrain, though it might be there at smaller length scales that are hard to identify. But there does seem that quite a lot of the cortex can be destroyed without loss of function, and thus offering some redundancy. It is the cortex where most of the brain activities occur concerning seeing, hearing, language and movement. Adjacent to it is the claustrum which, with its neural connections to most parts of the cortex, has been suggested by Crick and Koch as having a central role for consciousness [8]. Recognition of the continuing renewal of brain components (including structures in the claustrum and the cortex, and presumably also the “memory biochemicals” [9] has several consequences.
Consequences of Replacement and Renewal
The replacements of active components of the brain, whether cells or molecules, has to be done in a way that does not interfere with normal functions in an unacceptable manner. After all, one important brain function is being able to move away from dangers. It would not be good to run out of brain function when pursued by a tiger. However, we do sleep, and we do so at times and places over which we (usually) have some control, such that we are not exposed to these dangers. So I suggest Conjecture A:
The regular replacement of basic brain components takes place primarily when we slee
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