Sleep is a fundamental property of the mammalian brain. People with all manner of brain damage continue to sleep. No matter what happens to your brain the surviving undamaged part finds a way to sleep during part of the day.
Only the most ignorant think sleep is wasted time. Everyone with a modicum of introspection knows sleep is necessary. People try to cheat it - to shorten the time they spend asleep - but their efforts fall short. Their efforts include artificially restricting sleep or trying polyphasic sleep regimens.
Scientists have done tests on animals where sections of the brain are cut off from other parts, and all parts continue to go through something like sleep.
The evolutionary origins of sleep are very ancient and it is found across the animal kingdom. All animals, even insects sleep. Simpler animals don’t sleep anything like humans because they don’t have our brains and much of the interesting aspects of human sleep involve the brain. But insects have periods of low physical activity and seem to build up some sort of sleep debt – if forced to remain in the awake period longer than normal, they sleep longer when allowed to rest. Sleep is partly about physical rest, and it is believed mammalian sleep evolved from a period of metabolic quiet. Sleep does many things for us – or perhaps more accurately, the body and brain do many functions during the sleep period. Perhaps the most obvious is simple rest. We are relatively inactive while asleep and consumer fewer calories. After sleep we have more physical strength. Animals may find it easier to hide from predators while sleeping. In the “non-adaptive theory” of sleep origins, this was the basic reason animals slept. So even as our brains do many things during sleep, including important housecleaning important to learning and memory, sleep did not start out that complicated hundreds of millions of years ago. Sleep started to provide one function (rest and energy conservation), but picked up more functions as animals evolved.
This process is called exaptation. Functions and utilities the body and brain needed to provide moved to the sleep period because it was convenient to put it there.
Is Sleep Global or Local in the Brain?
BOTH!At one time it was assumed that the there was a central control in the brain that brought on sleep and waking. While there is a master clock – the SCN – the control is not top-down as it is in a computer. It appears that individual sections of the brain – neuronal assemblies – communicate together through chemical and electrical signals. They synchronize and the state of sleep can be thought of as an "emergent property" of the networks.
Brain scientists use cortical columns in many models. In this idea of sleep, the person is asleep it is because most of the cortical columns are in the sleep position.
So in addition to sleep being an emergent phenomenon, sleepiness may be an emergent phenomenon. If enough columns are in the sleep position, the person feels sleepy. But the columns that are in the sleep position are not necessarily the same every time the person gets sleepy. - the brain changes.