[ 2011-07-26 ]

Continuous sleep is important for memory formation

Fragmented sleep 'harms memory'
Broken sleep affects the ability to build
memories, a study of mice suggests.

The Proceedings of the National Academy of Science
findings could help explain memory problems linked
to conditions including Alzheimer's and sleep
apnoea.

The Stanford University found disrupting sleep
made it harder for the animals to recognise
familiar objects.

A UK sleep expert said the brain used deep sleep
to evaluate the day's events and decide what to
keep.

This study looked at sleep that was fragmented,
but not shorter or less intense than normal for
the mice.

It used a technique called optogenetics, where
specific cells are genetically engineered so they
can be controlled by light.

They targeted a type of brain cell that plays a
key role in switching between the states of being
asleep and being awake.

Mouse memory test

The researchers then sent light pulses directly
into the brains of mice while they slept.

This meant they could disrupt their sleep without
affecting total sleep time or the quality or
composition of sleep.

The animals were then placed in a box with two
objects, one of which they had encountered
before.

Mice would naturally spend more time examining the
newer object, and those who had been allowed
uninterrupted sleep did just that.

But those whose sleep had been disrupted were
equally interested in both objects, suggesting
their memories had been affected.

Writing in the journal, the researchers, led by Dr
Luis de Lecea, said: "Sleep continuity is one of
the main factors affected in various pathological
conditions that impact memory, including
Alzheimer's and other age-related cognitive
deficits."

Broken sleep also affects people addicted to
alcohol, and those with sleep apnoea - a condition
in which the throat repeatedly narrows or closes
during sleep, restricting oxygen and causing the
patient to wake up.

The researchers add there is no evidence of a
causal link between sleep disruption and any of
these conditions.

But they added: "We conclude that regardless of
the total amount of sleep or sleep intensity, a
minimal unit of uninterrupted sleep is crucial for
memory consolidation."

Independent sleep expert Dr Neil Stanley, a former
chairman of the British Sleep Society, said:
"During the day, we accumulate all these
memories.

"At some point we have to sort through what's
happened during the day.

"There are some things that we need to 'lock down'
as a permanent hard memory.

"That process occurs in deep sleep. So anything
that affects sleep will have an effect on that
process to a greater or a lesser extent."

Dr Stanley said there was particularly striking
evidence that people with sleep apnoea had
particular problems "locking down" memories.

And he added that people with Alzheimer's often
had trouble sleeping, but said: "There is
something there. But whether it's the degeneration
of the brain that causes poor sleep, or poor sleep
that aids the degeneration of the brain has not
been determined."

Source - BBC

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