Physiology of sleep & consciousness

  • By: Terri
  • Date: November 8, 2010
  • Time to read: 6 min.


    • reversible behavioural state of unresponsiveness & perpetual dissociation from the environment
    • Perception of sensations, voluntary initiation & control of movement, capabilities associated with higher mental processing

Stages of consciousness




  • Not a period of absolute rest
    • some parts of the brain are more active during sleep
    • Dreams
      • necessary
      • Experiments where people have been awakened as they enter the REM sleep phase become moody & depressed, and disoriented; and may have hallucinations
  • Purpose of sleeping
    • Restorative
      • especially slow wave sleep
    • Conservation of energy
    • Maintaining adequate function of the immune system
    • Mentally practise & refine behaviour
      • e.g. escaping from predators without actually having to perform them
    • REM sleep
      • we dream to forget
      • reverse learning helps us forget trivial things
      • and to sort out our emotions
  • Physiological changes in sleep
    • Decreased reaction to external stimuli
    • Reduced activity of the sympathetic system
      • Parasympathetic dominance
      • Reduced heart rate, BP, respiratory rate
        • especially in Non-REM sleep
      • Reduced metabolic rate
        • Somnolent Metabolic Rate (SMR)
          • is lower than BMR
    • Reduced muscle tone
      • Extensor  Babinski response
        • unmasking of extensor dominance
    • Changes in EEG
    • Increased growth hormone production during deep sleep
      • decreased in REM sleep

Sleep cycles



  • First cycle:
    • descends from stage 1-2-3-4
    • resurfaces from 3-2-1
    • enters REM-descends intom2-3-4 and so on
  • Subsequent cycles become shorter
  • The cycle ends with REM sleep
  • In adults, sleep begins with NREM
    • in infants it begins with REM
    • REM-onset sleep is seen in adults in
      • jet lag
      • chronic sleep deprivation
      • acute withdrawal of REM-suppressing drugs
      • endogenous depressinarcolepsyon
  • Retrograde amnesia
    • poor recall of midnight dreams
    • not remembering clock ringing




  • Awake
    • Alert
      • Beta wave
    • Resting
      • Alpha wave
  • Sleep
    • Stage 1
      • Theta wave
    • Stage 2
      • Spindles
      • K complex
    • Stage 3
      • Delta wave appears
    • Stage 4
      • Delta wave
        • Also seen in deep sleep/ anaesthesia / brain damage in awake adults
      • Synchronisation
    • REM Sleep
      • Ponto-geniculo-occipital (PGO) spike


Difference between deep sleep and REM sleep


Changes in pattern of sleep with age



  • With age
    • REM sleep decreases
    • deep sleep decreases
  • Total sleep time decreases

Regulation of sleep

  • Sleep is regulated by
    • an interaction of homeostatic and circadian processes
    • The homeostatic mechanisms
      • keep track of how long we have been awake and asleep
      • how tired we are!
    • The circadian process
      • determines the optimal time for sleep
  • They influence sleep duration and the relative contribution of the two major types of sleep:
    • non-rapid eye movement (NREM)
    • rapid eye movement (REM) sleep

The circadian process

  • Circa=about , dian=day
  • The suprachiasmatic nucleus (SCN) dictates the sleep-wake cycles over a roughly 24-hour period
    • the SCN receives inputs
      via the retino-hypothalamic fibres
      • is  influenced by external cues (e.g daylight)
      • but the rhythm is still present in volunteers kept in caves or in people blind from birth
      • Light initiates sleep in nocturnal animals & waking in diurnal animals (including humans)
  • Melatonin & melanopsin may play a role
    • Phase shifts can occur with rapid crossing of time zones
      • resulting in ‘jet lag’

The ultradian process

    • that is characterized by the alternation of the two basic sleep states non-rapid-eye-movement (NREM) sleep and REM sleep
      • influenced by SCN input


  • Nucleus reticularis pontis oralis – have 4 nuclei
    • PGO-on
      • causes PGO spikes associated with REM sleep
    • REM-on (stimulatory)
      • atonia of muscles during REM sleep
    • REM-on (inhibitory)
      • inhibits the 2 nuclei inhibiting PGO-on
    • REM-waking on
      • eye movements
      • muscle twitches
  • Hypothalamus – have 1 nuclei
    • NREM-on


Sleep disorders

  • Transient relief with “sleeping pills”
    • benzodiazepines
    • but daytime performance may be compromised


  • Definition
    • chronic inability to maintain the amount/quality of sleep to function normally in the day
    • due to medical or mental conditions
  • Right amount of sleep
    • sleep requirements vary from 4-9 hours between individuals
    • Margaret Thatcher & Napoleon need(ed) only 4 hours of sleep
  • Fatal familial insomnia
    • autosomal dominant inheritance
    • Features
      • worsening insomnia
      • impaired ANS & motor functions
      • dementia
      • death
  • Types of insomnia
    • Onset sleep insomnia
      • difficulty in falling asleep initially
      • associated with anxiety disorders
    • Maintenance sleep insomnia
      • wake up middle of the night
      • difficulty in remaining asleep
        • with frequent awakening at night
      • may get back to sleep
      • associated with
        • pain disorders
        • medical illness
    • Terminal (late) insomnia
      • persistent early morning awakening
      • cannot get back to sleep
      • characteristic of clinical depression

Sleep deprivation

  • Marked by brief psychosis but non-permanent psychological effects
  • Features
    • irritability
    • stimuli misperception
    • decreased waking alpha activity
    • disorientation
    • lack of attentiveness
    • Neurological consequences
      • tremors
      • seizures


  • Definition
    • a chronic neurological disorder caused by the brain’s inability to regulate sleep-wake cycles normally
      • excessive daytime sleepiness (EDS)
  • Narcolepsy results from
    • disease processes affecting brain mechanisms that regulate REM sleep
  • For normal sleepers a typical sleep cycle is about 100 – 110 minutes long, beginning with NREM sleep and transitioning to REM sleep after 80 – 100 minutes
    • But, people with narcolepsy frequently enter REM sleep within a few minutes of falling asleep
  • Three other major symptoms frequently characterize narcolepsy
    • Cataplexy
      • the sudden loss of voluntary muscle tone
    • Vivid hallucinations
      • during sleep onset or upon awakening
    • Brief episodes of total paralysis
      • at the beginning or end of sleep 

Somnambulism (sleep walking)

  • more common in
    • children
    • males
  • may last a few minutes
  • walk with eyes open
    • to avoid collision
    • but cannot remember anything
  • Occur during arousal from slow wave sleep
    • somnambulism
    • noctural aneuresis
      • bed-wetting
    • night terrors
  • Nocturnal emission
    • Wet-dreams
    • normal in male (sometimes females) of reproductive age

Sleep apnoea

  • Definition
    • stopping of breathing for short periods
      • typically between 10 and 90 seconds) during a normal nightly sleep
  • Can be repeated hundreds of times during a sleep period
  • Almost always accompanied by
    • snoring, which is usually quite loud
  • Patient is  unaware of this happening, in normal circumstances
  • 3 types of sleep apnoea
    1. Obstructive sleep apnoea (OSA)
      • Fairly common
      • upper airway collapses as the throat muscles relax during normal sleep
      • cerebral hypoxia results in waking up
        • may be repeated every 15 minutes
      • May result in
        • excessive daytime tiredness and a lack of concentration
      • Counter-measure
        • Relieve the obstruction
          • sleep on the side; reduce weight
    2. Central
      • Rare
      • Neurological dysfunction
        • the brain simply "forgets" or "delays" to send the signal to breathe
      • More difficult to treat
        • some drugs may help
    3. Mixed
      • Extremely rare


Reticular Activating System (RAS)


  • The reticular formation resides in the brain stem
    • midbrain, pons & medulla
  • Comprised of the
    • motor system (reticulospinal system)
    • sensory systems (RAS)
    • the vital centres
  • The RAS stimulates the whole brain to cause arousal (wakefulness)
    • its output is suppressed
      in sleep & coma
    • stimulated by
      • input from the cortex (corticofugal fibres)
      • OREXINS
        • peptides released by hypothalamic neurons just before waking




  • Various definitions
    • mind
    • awareness
    • ability to respond
  • Can it be equated to activity in the EEG?
  • Sometimes, people under general anaesthesia feel the pain and are conscious of their surroundings
    • but cannot convey this to the outside world , trapped in their own body
  • Perception of sensations, voluntary initiation & control of movement, capabilities –> associated with higher mental processing
  • The brain’s ability to adjust its activity and consciousness levels can be impaired in several ways:
    • When people are severely deprived of sleep
    • When and immediately after a seizure occurs
    • When both cerebral hemispheres are suddenly and severely damaged
    • When the reticular activating system malfunctions
    • When blood flow or the amount of nutrients (such as oxygen or sugar) going to the brain decrease
    • When toxic substances impair the brain (liver & kidney failure)
    • Drugs
      • morphine
      • diazepam
      • antipsychotic drugs
  • Periods of impaired consciousness can be short or long. The level of impairment can range from slight to severe:
    • Lethargy
      • slight reduction in alertness or clouding of consciousness
      • People tend to be less aware of what is happening around them and to think more slowly
    • Obtundation
      • moderate reduction in alertness or clouding of consciousness
    • Stupor
      • excessively long or deep sleep-like state
      • A person can be aroused from it only briefly by vigorous stimulation
        • such as repeated shaking, loud calling, pinching, or sticking with a pin.
    • Coma
      • state of complete unresponsiveness
      • A person cannot be aroused at all
      • A person in a deep coma lacks even the most basic responses, such as avoidance of pain
        • although reflexes may be present

(read coma lecture notes)

  • In coma, brain oxygen utilisation is below normal resting levels
    • in sleep, the brain is active and oxygen consumption is comparable to the waking state
  • Caused by
    • trauma
    • tumours
    • metabolic disorders
      • hypoglycaemia (hyperosmolar coma)
      • drug overdose
      • liver and kidney failure
      • fluids & electrolytes imbalance
        • hypernatraemia
        • hypokalaemia
        • water intoxication
      • myxoedema
  • Depression of the reticular activating system (RAS)
  • Clinically assessed using Glasgow Coma Scale (GCS)
  • Four Score Coma Scale
      • eye & motor responses
      • brain stem reflexes
      • respiration.
        Total unresponsiveness to sensory stimuli for an extended period

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