The easiest response to the question “why do we need to sleep?” is simply that “we get tired.” In truth this is the best explanation we have. Obviously people perform better in a variety of mental and physical tasks when they are not tired and sleep has definite benefits relating to the physiological effects it has on us (lower body temperature, faster healing of wounds etc). It has been shown that rats forced to not sleep eventually died; but exactly what killed them? Whether it was the breakdown of cells or an infection or perhaps their rapid drop in body temperature is still unknown, meaning that we don’t know what exactly about sleep keeps us alive.
Brain plasticity is simply the ability for the brain to change and reform based on new information. It is well known that sleep aids in this. When we sleep the data our brains have been gathering over the course of the day gets collected and stored and sometimes is replayed as a dream. The two main states of sleeping, rapid eye movement and non-rapid movement, are represented by different electroencephalographic readings (EEG, using electrodes along the scalp to record brain activity) but exactly how these states affect the storing of long term memories is not understood.
One of the key features of the EEG graph is called a sleep spindle which occurs occasionally. This spindle is an oscillation of voltage in the brain with a frequency of about 9 to 16 Hz for a sigma (σ) wave and 16 to 30 Hz for a beta (β) wave, each lasting for a couple of seconds. These spindles have been suggested as also important when it comes to brain plasticity.
A dendrite is the spreading tree like projection from an axon and is the primary method of synaptic communication in the brain (see this post as well as the diagram above). It has been shown through experiments on more rats that a lack of sleep hampered the “pruning” and restructuring of the dendrites while sleep eased it. To study this in more detail electroencephalographic results and measurements of the calcium ions (Ca2+) were taken focusing on the layer V neurons shown above (second from the left).
It was found that increased activity of the calcium ions in the dendrites correlated with activity of the sleep spindles both σ and β. It is therefore concluded that this synchronisation between calcium and the spindles is the physiological mechanism of how memories can be stored. In other words the spindle represents an increase in brain activity and the calcium activity in the dendrites is the physical change occurring in the neuron to facilitate the memory being stored. Ultimately to elucidate the process more focus needs to be held on the layer V neurons as they are clearly significant for memory development.