Local versus global sleep organization and the quest to determine sleep function

The research of JM Krueger and colleagues, focusing on sleep organization as a means to elucidate sleep function, led to critical insights as to why we sleep. Krueger posited that, fundamentally, sleep occurs locally at the level of neuronal/glial assemblies (small networks of neurons and glia) and that the expression of sleep in these assemblies is dependent on their prior use. Neuronal/glial assemblies serve as units of information processing, which consumes energy and increases entropy so that the energy available for further information processing is use-dependently depleted. According to the laws of physics, when energy drops to a lower bound relative to entropy, information processing ceases – which results in local quiescence and locally reduced consciousness and manifests as use-dependent local sleep. The physics-based nature of local sleep implies that it is inevitable, has neither function nor purpose, and is by itself not subject to biology-based evolutionary shaping. But uncontrolled local sleep compromises vigilance and is a threat to safety, which needs to be addressed to ensure survival. This can be accomplished by preemptively regulating sleep at a more global level and in a way that is adapted to the organism's temporal, environmental and ecological niche. Such global sleep allows for energy resupply (through biological processes not unique to sleep) across many neuronal/glial assemblies simultaneously while the organism is relatively safe. Thus, global sleep regulation could be the biology-based adaptation to the physics-based problem of use-dependent local sleep intrusions into wakefulness. Global sleep precludes niche exploitation and thus comes at an opportunity cost – but, unlike local sleep, the regulation of global sleep is subject to evolutionary shaping and amenable to species-specific optimization. Furthermore, a variety of ancillary functions may be served during global sleep to retroactively address biological needs that arose from prior wakefulness. However, serving these functions may be merely opportunistic, as the temporal dynamics of global sleep regulation appear to be proactive rather than retroactive, prioritizing alignment of global sleep and wake timing with the organism's ecological niche. Regardless, the costs of use-dependent local sleep and the management thereof through global sleep regulation are likely to be outweighed by the evolutionary benefit of the presumed source of the local sleep problem – that is, information processing capability, or cognition. In essence, therefore, sleep may just be the unavoidable, but worthwhile, price we pay for cognition.