Sleep has profound importance in our lives, such that we spend a considerable proportion of our time engaging in it. Sleep enables the body, including the brain, to recover metabolically, but contemporary research has been moving to focus on the active rather than recuperative role that sleep has on our brain and behaviour.
Sleep is composed of several distinct stages. Two of these, slow-wave (or deep) and REM sleep, reflect very different patterns of brain activity, and have been related to different cognitive processes.
Slow-wave sleep is characterised by synchronised activity of neurons in the neo-cortex firing at a slow rate, between 0.5 and three times per second. The neo-cortex comprises the majority of the cerebral cortex in the brain which plays a role in memory, thought, language and consciousness. In contrast during REM sleep, when most of our dreaming happens, neuronal firing is rapid and synchronised at much higher frequencies, between 30 to 80 times per second.
Such patterns of brain activity during REM sleep are reminiscent of those observed during wakefulness, and for this reason REM sleep is often referred to as “paradoxical” sleep.
There is growing evidence that slow-wave sleep is related to the consolidation of memory and is involved in transferring information from the hippocampus, which encodes recent experiences, and forging long-term connections within the neo-cortex. REM sleep has been linked to processes involving abstraction and generalisation of experiences, resulting in creative discovery and improved problem solving.
Though there are substantial similarities between wakefulness and REM sleep, numerous studies have explored differences in the activity of brain regions between these states, with the cingulate cortex, hippocampus and amygdala more active during REM sleep than wakefulness. These regions are particularly interesting to cognitive neuroscientists because they are key areas involved in emotional regulation and emotional memory.
However, which sub-regions are active within these broader cortical and limbic areas – the pathways in the brain that produce these patterns of activation – and the precise function of the activity in these regions during REM sleep is currently under-described.
Cortical Activity in Rats
A new study published in Science Advances studied the physiology and functionality of REM sleep in a group of rats and provides insight into the cortical activity and the sub-cortical pathways that result in this activity. The level of detail of this study provides a major step forward for our understanding of the effect that REM sleep has on our brain and cognitive behaviour.