The relationship between sleep and
learning-memory processes
For almost a century, several studies showed the
beneficial effects of sleep on memory functioning
in animals and humans for different types of
learning materials.9,27,28 Recent studies in molecular
genetics, neurophysiology, and cognitive and
behavioural neuroscience have strengthened the
idea that sleep may play an important role in
learning and memory, although the extent of this
role remains hotly debated.29 In fact, there is still
poor understanding as regards which aspects of
memory function are affected by sleep and which
processes underlie memory consolidation. Moreover,
it is not clear whether memory consolidation
is linked to a particular sleep stage, and whether
different types of memory (e.g. declarative,
procedural) are differentially influenced by sleep
stages.
Now human memory is divided into at least two
branches: declarative and procedural memory.
Procedural knowledge comprises memories of how
to perform some skill or how to solve a problem
(‘knowing how’). These memories, which may
pertain to the motor, visual or even verbal
domain,30 are usually unconsciously learned31 and
are often referred to as ‘non-declarative’. Declarative
material refers to memories accessible to
conscious recollection (‘knowing that’). This distinction
led toward the dual process hypothesis:9
the effect of sleep state on memory process would
be task-dependent, with the procedural memory
gaining from REM sleep10 and declarative memory
linked to NREM sleep.32