It has been suggested that sleep is of particular importance to brain processes such as memory consolidation and learning. Experimental studies have supported this ‘memory consolidation’ hypothesis of sleep function by showing that sleep-deprived human and animal subjects perform poorly in learning tasks when compared to individuals that are well rested. However, the approach and conclusions of these studies are often criticized, due to the stress associated with sleep deprivation experiments and because memory consolidation can also occur in the absence of sleep.
Ecologically-imposed needs for increased memory capacity should be reflected by an increase in the size of brain structures that are responsible for memory processing and consolidation. For example, spatial memory is important in animals that hoard food because it improves their ability to retrieve stored food at a later time, which in turn enhances fitness. The hippocampus is one of the most important brain structures involved in spatial memory processing and memory retention, and studies in birds have shown that hippocampal volumes and hippocampal neuron numbers are higher in species and populations that cache food relative to those that do not exhibit such behavior. Similarly, if sleep serves a specific function with regard to memory consolidation and learning, we expect that greater memory-related demands result in a greater need for sleep. Those brain structures that are devoted to memory processing and learning therefore should be positively associated with sleep durations.
Many brain regions are involved in the diverse aspects of memory formation, but those hypothesized to have prominent roles in forming adaptively relevant associations in mammals include the hippocampus, amygdala and neocortex. During sleep a variety of brain structures are in a highly activated state, including those specifically linked to memory consolidation and learning. Interactions of the amygdala and hippocampus with one another and the neocortex during NREM sleep are well documented, while the outflow from the hippocampus to the neocortex is inhibited during REM but not NREM sleep. Recent studies converge on the conclusion that procedural and emotional forms of memory benefit from both REM and NREM sleep, while episodic memory benefits only from NREM sleep.
The researchers - Isabella Capellini, Patrick McNamara, Brian T. Preston, Charles L. Nunn, Robert A. Barton - hypnotized that sleep is involved in memory consolidation and learning processes. That leads to a prediction that evolutionary increases in the relative size of mammalian neocortical, hippocampal, and amygdalar regions will be associated with increased durations of REM and NREM sleep. Researchers found that increases in NREM sleep durations were correlated with increases in the size of the amygdala, and this effect was independent of both scaling effects and phylogeny. But found no evidence of positive relationships between REM sleep and brain structures implicated in memory consolidation and learning. The hippocampus showed a tendency to increase with NREM sleep, although this relationship was not statistically significant. The study nonetheless reveals the first evidence of correlated evolution between sleep and specific brain structures, and provides support for the idea that memory consolidation may be among the ultimate functions of sleep.
Other aspects of sleep architecture are likely to be important in sleep function and evolution, particularly sleep intensity. Slow-wave activity during NREM sleep is considered to be a measure of sleep intensity and is implicated in the homeostatic regulation of sleep; sleep deprived subjects experience increases in slow-wave activity as well as the duration of subsequent sleep periods. Thus, memory related demands of sleep might also be met by an increase in sleep intensity. Unfortunately, there are insufficient comparative data currently available to investigate how sleep intensity may interact with sleep durations and brain structure in generating the benefits of sleep.
Brain image taken from wiki.