Sleep and memory
In this review we focus on the role of sleep in learning and memory, especially memory consolidation. The arousal state of humans and other mammals varies according to the circadian rhythm regulated by light. In humans, nightly sleep stages repeat in a cyclic manner so that non-rapid eye movement (non-REM) sleep is more common first and REM sleep is more common towards the morning. Compared to awake state, sleep improves memory consolidation. During non-REM sleep, neuronal activation patterns formed during awake experience repeat in the brain. This process is thought to reflect the consolidation of information into long-term memory in distributed neural networks in the neocortex and sub-cortical structures like the thalamus and the hippocampus. The role of REM sleep in this systemic-level mechanism of memory consolidation is still unclear. REM sleep appears important for learning as its amount and proportion increases after learning and is reflected in the memory performance quantified after sleep. Sleep strengthens the consolidation of both procedural and declarative memory traces. It is also possible that one can learn about stimuli presented during sleep, and reminders of previous learning presented during sleep improve memory consolidation. The role of sleep in learning and memory is thus crucial and sleep can be utilized in improving learning.