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Is sleep 'hard-wired' into the brain?

November 25, 2002

Falling asleep is usually thought of as something we can control ourselves as part of our behaviour patterns. In a new article in the December Journal of the Royal Society of Medicine, Dr Bidi Evans argues that waking and sleeping is actually controlled by a physical mechanism that is 'hard-wired' into the brain. She suggests that evidence from people with brain damage shows that coma patients literally cannot wake up, because their waking mechanism is 'broken'. Dr Evans recommends that sleep medicine, usually studied by psychiatrists and psychologists, "should enter the mainstream of neurological thinking" where more neural specialists could contribute to research.

What we know about sleep
Waking up: The waking up process was first described fifty years ago. The 'reticular activating mechanism' (RAM) starts in the nerve cells around the central canal and switches the brain from deep sleep to wakefulness.
Falling asleep: Instead of being a passive process, as originally thought, falling asleep may be an active one - the 'arousal inhibitory mechanism' that stops us from being conscious.

How we wake up in an emergency
Every four seconds in dreamless sleep there is a short burst of activity deep in the brain known as a 'sleep spindle'. The burst interrupts activity in the brainstem and combines with slower waves in the cortex to actively stop the RAM from re-activating the brain. Since the spindle occurs intermittently, if we need to wake up - for example to care for a crying baby - we still can. Dr Evans argues that deep sleep is a kind of "reversible unconsciousness".

How brain damage can stop the brain from waking
Dr Evans suggests that since the sleep system involves three different levels of the brain, damage to any or all of these levels can mean that the brain can no longer wake itself up.
Level 1: the cerebral hemisphere. If this part of the brain is damaged (eg by meningitis , Alzheimer's) then the RAM cannot activate the cortex, so the patient does not fully wake up. People with level 1 damage still have 'sleeping' and 'waking' periods in the brain, but in the waking periods the cortex does not allow them to be properly aware. This is sometimes known as persistent vegetative state.
Level 2: the thalamus and upper mid brain controls the intermittent mechanism that allows us to be woken up when necessary. Damage here (eg by stroke, tumours) can cause spasms and extreme changes in cerebrospinal fluid pressure. The patient will be unconscious but may suddenly start to breathe hard with an increase in heart rate.
Level 3: the lower mid-brain and upper pons is where most of the sensory input to the brain is received, so if you are damaged here, even strong stimuli will not wake you up.

Dr Evans compares the sleep processes she describes with other neural mechanisms such as vision, which is affected if the optic nerve or related fibres in the brain are injured. "If the sleep mechanisms are damaged, the result will be unconsciousness, i.e. coma."

Royal Society of Medicine

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