Sleep

• Theories of the purpose of sleep

  • Restorative function
  • Evolutionary and enhances survival by being inactive at night
• Evolutionary purpose of sleep


  Sleep evolved to enhance survival by protecting an organism by making it inactive during the part of the day when it is most risky to move around
• Survival theories of sleep

  1. Organism fulfills survival functions

  2. Conserves energy

  3. Hides and is protected from predators

  4. Does not interact with environment
• Sleep serves the function of protecting the sleeper from harm or death


  Enhances the survival of the species
• The need for sleep at a particular time

  Forces the animal to conform to the ecological niche it was adapted for (nocturnal vs diurnal)
• Research evidence for survival theories

  • Studies on behaviour patterns and sleep-wake cycles of different species
  • Animals with few predators such as lions sleep as much as 15 hours a day
• Humans sleep at night because we are highly visual and need light to find food and do other things necessary for survival
• Humans are NOT well adapted to searching for food in the dark or protecting ourselves from nocturnal predators
• During most of our evolution, we would be tucked away in a cave protecting ourselves from predators and the cold nights
• Our pattern of sleep is most likely a carryover from this time when night was a time of great danger
• A criticism of survival (evolutionary theories) is that it doesn't explain why sleep involves a loss of consciousness or awareness- as this may place the organism at greater risk
• There is limited evidence to this perspective and psychologists have no definite answers as to why we sleep
• They do know there are multiple purposes- two of which are survival and restorative
• Restorative theories (or recovery theories)


  Propose that sleep allows the body to replenish the stores of energy to help us recover from depleting activities during waking time that use up the body's physical and mental resources
• Restorative theories

  • Allow damaged cells to be repaired
  • Allow muscles to be detoxified or rid themselves of waste products
• People usually sleep for longer periods when they are sick

  Suggests that sleep may have something to do with the recovery process
• When we sleep our body functions slow down (breathing, heart rate, neural activity)
• Growth hormones that promote repair are secreted at a much higher rate when asleep than when awake
• Studies involving sleep-deprived rats have shown that prolonged sleep deprivation results in the breakdown of bodily tissues (e.g. skin sores start to heal) and death within 3 weeks
• Restorative functions of NREM and REM sleep

  Have different restorative effects
• NREM-

  • is important for restoring and repairing the body

  REM-

  • Restores the brain and higher mental functions such as learning and memory. REM is much more abundant in the developing fetus and infants compared to childhood and later stages of development. This indicates that REM plays an important role in the peak periods of brain development that occur in infancy
• REM

  Maintains brain circuits between groups of neurons
• Synapses


  Can deteriorate if they go too long without being active
• Increased brain activity observed during REM

  Preserves neural pathways
• REM sleep

  Consolidates the memory of newly learned information by strengthening neural connections that form during the learning process
• REM rebound


  1. Person is woken up several times during REM sleep

  2. They will eventually spend more time in the REM stage when they eventually have uninterrupted sleep

  3. Catching up on REM sleep immediately following a period of lost REM by spending more time in the REM stage when next asleep
• Researchers still have not established exactly what is restored, repaired, or revitalised during REM sleep
• Sleep-Wake Cycle

  • Over the course of the nights sleep we experience two different types of sleep-known as NREM (Non-rapid eye movement) and REM (rapid-eye movement)
  • These occur in a continuous cycle with one following the other
  • In adults, one cycle of NREM sleep lasts about 70-90 minutes, and consists of 4 distinct stages, each of which can be identified by a different brain wave
  • A period of REM sleep follows each period of NREM sleep, and researchers sometimes refer to REM sleep as the fifth stage of sleep
• NREM Sleep

  Approximately 80% of our sleep time is spent in NREM - typically the first half of the night has more NREM sleep than the second half of the night
• NREM sleep


  • Brain is active, but not as active as the REM sleep
• NREM sleep stages

  Sleeper progresses from a stable of light sleep to the stage of deepest sleep and back again through one or more stages to light sleep
• EEG (Electroencephalograph)


  Psychologists can determine an individual's stage of sleep by examining EEG data
• Brain wave patterns/rhythms


  • Each stage is dominated by a particular identifiable brain wave pattern/rhythm that is different from that of the other stages
• Frequency


  Used to describe the number of brain waves
• Amplitude


  Used to describe the intensity of the brain waves (estimated by the size of the brain waves- peaks and troughs)
• Alpha waves


  • Associated with relaxation and drowsiness- the pattern is relatively high frequency and medium amplitude waves
• Hypnagogic state


  1. Transitions period from being awake to being asleep, characterised by slow, rolling eye movements for a minute or two

  2. Some people experience flashes of light or colour, feelings of floating and weightlessness, dreamlike images, jerky movements or a sense of falling or slipping

  3. This is the beginning towards the first stage of sleep in the NREM