An introduction to sleep

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An introduction to sleep
Psychobiology NoH 2006
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What is sleep?

• We spend a third of our lives asleep
• Still not clear what it is for!
• Sleep is defined behaviourally as
• Reduced motor activity
• Decreased response to stimulation
• Stereotypic postures
• Relatively easy reversibility

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Characteristics of sleep

• Sleep is a circadian rhythm, controlled by the
  biological clock
• We spend 30 of our lives asleep

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SWS Slow Wave Sleep aka NREM sleep

• Cerebral blood flow declines to ¼
• Reduced muscle tone
• Decreased heart rate, blood pressure, respiration
  and body temperature
• Gastrointestinal activity might increase

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REM

• Blood flow to cerebral cortex similar to when
  awake, brain temperature increased so REM
  shares similarities with the waking state
• The brain self stimulates with PGO (pontine
  geniculate occiptial) spikes
• Paralysis of the muscles apart from periodic
  twitches
• Paradoxical sleep- brain seems awake, but body is
  paralysed
• Increased heart rate, blood pressure, breathing,
  erections
• Dreams- often bizarre storylines

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Methods of Studying sleep

• EEG (electro encephalogram) - Studies brain wave
  activity, firing of the neurons
• EMG (electro myogram) - Studies muscle activity
• EOC (electro oculogram) - Studies eye movement
• Lesions (animal studies)
• Insertion of wires into parts of the brain
• Levels of neurotransmitters in spinal fluid
• Developmental (over lifespan)
• Comparative (compare with different animals)

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• Electroencephalogram (EEG) wavesStages of sleep

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Healthy Sleep Cycle

• Four to five 90 min sleep cycles/night
• 80SWS, 20REM
• REM increases gradually from 20 mins to 40 mins
  during the cycles

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Sleep structure differs across the lifetime
Data from Roffwarg et al., Ontogenetic
development of the human sleep-dream cycle,
Science, 152604-619, 1966, from
http//faculty.washington.edu/chudler/sleep.html
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How do we fall asleep?

• Non REM sleep is generated by neurons in the
  basal forebrain and medulla interacting with
  neurons in the midbrain and diencephalon
• REM sleep is generated by neurons in the caudal
  midbrain and pons interacting with neurons in the
  medulla and forebrain

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Light/dark information from visual pathways
NB arousal causes wakefulness via activation of
the RF which overrides the hypnogenic centres of
the brain stem
Retina
Pineal gland (secretes melatonin)
Suprachiasmatic nucleus (Part of the hypothalamus)
Regulates REM and SWS
Locus cooeruleus (in the pons, produces
noradrenaline and acetylcholine
Raphe nuclei (lies alongside the RF, produces
serotonin)
Hypnogenic Centres in the Brain stem
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Circadian rhythms

• Sleep involves both neural and chemical
  mechanisms
• Circadian rhythms are regular changes in mental
  and physical characteristics that occur over a
  24hour period. Most circadian rhythms are
  controlled by the suprachiasmatic nucleus
  internal clock (SNC) a tiny set of brain
  structures in the hypothalamus the size of a
  pinhead and containing just 20,000 neurons.

• Many circadian rhythms have a periodicity of 24
  hours
• Feeding, foraging, homeostatic regulatory
  mechanisms
• E.g., regulation of body temperature
• Endogenous rhythms

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• Signals from the SNC travel to several brain
  regions, including the pineal gland, which
  responds to light-induced signals by switching
  off production of the hormone melatonin. The SNC
  also affects functions which are synchronised
  with the sleep/wake cycle, including body
  temperature, hormone secretion and changes in
  blood pressure.

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What is sleep for

• Probably different functions for REM, SWS and
  Dreaming.
• May be more than one function.
• Certainly, we are in trouble if we dont sleep
  e.g. sleep deprivation psychosis
• One idea is the temperature regulation hypothesis

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Temperature regulation hypothesis.

• Non REM on cells are thermosensitive
• Is there a functional link with thermoregulatory
  processes? Wehr (1992)
• Body and brain temperature reduced in NREM
• Heating hypothalamus induces sleep in animals
• Cooling the hypothalamus induces wakefulness
• Fevers lead to more SWS
• Heating body prior to sleep increases SWS in
  humans
• Rats deprived of sleep show increases of 10
  degrees in preferred ambient temperatures

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More evidence for thermoregulation

• Hibernation entered from NREM
• shallow torpor from NREM
• During NREM core temp drops, REM starts at lowest
  temperature and warms core
• REM is only in warm blooded

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Further thermoregulation evidence

• Environment cold, and cold whilst in SWS, so
  blood flowing to hypothalamus is cold, which
  triggers REM
• If too cold ? wake up
• Behavioural warmers are off in REM, other factors
  warm CNS core
• At birth core body temperature is low and REM is
  high, often go straight into REM

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Amount of sleep Can you think of reasons for
this???
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Evolutionary

• Need for and difficulty of finding food
• Need for a safe sleeping place
• Risk of predation

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Restorative

• Memory consolidation REM, Empson and Clarke
  (1970), Jouvet (1978).
• Cognitive problem solving REM, SWS and dreaming
  may all have slightly different functions and
  contributions. Greenberg et al. (1972) -
• Repair SWS physical, REM brain and
  neurotransmitters

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Stimulation

• REM sleep could be a way of making sure the brain
  doesnt shut down completely during sleep
  Vertes and Eastman (2000).
• Prepares the brain for waking
• Allows the brain to periodically sample the
  environment for danger

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Do we all dream?

• Almost all animals have REM sleep and dreaming
• Stage 2 sleep is also associated with dreaming,
  however dreams during this stage are more
  concerned with thinking through problems -
  Kekule.

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Why do we Dream?

• Freud
• Activation synthesis hypothesis Hobson and
  McCarley
• Reverse Learning theory Crick and Mitchison
  (1983)
• Memory consolidation hypothesis

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REM deprivation and rebounds

• Increases in daytime sleepiness
• REM rebound after suppression
• But not the total amount
• Antidepressants
• suppress REM

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An overview of similarities and differences
between wakefulness and REM

• REM and waking state have similar EEG patterns
• Both exhibit mental activity
• Both exhibit a level of sensory processing
• There are different inputs to the system
• Some differences in neural substrates

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EEG in REM and Waking state

• Desynchronised, low voltage, fast activity EEG
• Hippocampal EEG is highly synchronised
• EEG gamma activity
• Associated with consciousness and waking states
• Attention and vigilance
• Also in REM but not SWS

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Sleep disorders

• Sleep disorders include problems with falling or
  staying asleep, falling asleep at inappropriate
  times, problems with adhering to a regular
  sleeping schedule and sleep disruptive behaviours

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Some sleep disorders

• Insomnia
• Obstructive sleep apnoea
• Narcolepsy http//www.devilducky.com/media/8232/
• Sleep walking and talking
• REM sleep behaviour disorder
• Like Jouvets cats
• Sleep paralysis
• Night terrors

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Summary

• REM and NREM sleep are different states
• Sleep (particularly NREM) is related to
  thermoregulation
• Sleep is also important for restoration and
  repair
• REM is similar to wakefulness and is related to
  stimulation of the cortex and CNS preparation for
  a potential fight, flight response
• Sleep probably has many functions
• Much of the same evidence is used to support all
  theories