Sleep

• Purpose of sleep


  Restorative function and evolutionary purpose
• Theories of sleep


  • Restorative theories
  • Survival theories
• Survival theories


  • Enhance survival by protecting an organism
  • Making it inactive during risky times
• According to survival theories, once an organism has fulfilled its survival functions, it must spend time conserving energy
• While sleeping, an organism does not interact with its environment
• Ecological niche


  Adaptation to nocturnal vs diurnal activity
• Research evidence for survival theories comes from studies of behaviour patterns and sleep-wake cycles of different species
• Lions sleep for about 15 hours a day while zebras sleep for 4 hours
• Humans sleep at night because we are highly visual and need light to find food
• During most of our evolution, we would be tucked away in a cave protecting ourselves from predators and cold nights
• A criticism of evolutionary theories is that it does not explain why sleep involves a loss of consciousness or awareness
• There is limited evidence for the evolutionary perspective and psychologists have no definite answers as to why we sleep
• Restorative theories


  Propose that sleep allows the body to replenish energy and recover from depleting activities
• Sleep allows damaged cells to be repaired and muscles to detoxify
• People usually sleep for longer periods when sick
• GH (growth hormone) which promotes repair is secreted at a much higher rate when asleep than awake
• Prolonged sleep deprivation of rats results in the breakdown of bodily tissues and death within 3 weeks
• NREM sleep


  • Important for restoring and repairing the body
• REM sleep


  • 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
• REM sleep consolidates the memory of newly learned information by strengthening neural connections
• REM rebound occurs when a person spends more time in REM stage after being woken during REM sleep
• Over the course of the night, we experience two types of sleep: NREM and REM
• One cycle of NREM sleep lasts for about 70-90 minutes and consists of 4 distinct stages
• A period of REM sleep follows each period of NREM sleep
• Approximately 80% of our sleep time is spent in NREM
• During NREM, the brain is active but not as active as during REM sleep
• NREM sleep consists of 4 stages from light sleep to deepest sleep
• Psychologists can determine an individual’s stage of sleep by examining EEG data
• Each stage of sleep is dominated by a particular identified brain wave pattern
• Frequency


  Describes the number of brain waves
• Amplitude


  Describes the intensity of the brain waves
• When we first close our eyes, we have a burst of alpha waves associated with relaxation
• Hypnogogic state


  Transition period from being awake to being asleep
• In NREM Stage 1, we lose awareness of ourselves and surroundings
• NREM Stage 1 typically lasts 5-10 minutes
• NREM Stage 2 is a light stage of sleep that lasts about 10-20 minutes
• NREM Stage 3 is known as deep sleep and is harder to wake someone up
• Stage 3 sleep is critical to restorative sleep, allowing for bodily recovery and growth
• Most time in deep sleep occurs during the first half of the night