Physiological Functions of Sleep Before we begin: Brainstorm 3 different functions of sleep.
Outline • Functions of Sleep- 6 Hypotheses (evidence for and against) • Hypothesis 1: Restoration and recovery of body systems • Hypothesis 2: Energy conservation • Hypothesis 3: Memory consolidation • Hypothesis 4: Protection from predation • Hypothesis 5: Brain development • Hypothesis 6: Discharge of emotions • Evolution of sleep • Sleep postures • Sleep places
Functions of Sleep • Sleep is essential for survival • In humans, extreme sleep deprivation will cause paranoia and hallucinations in otherwise healthy individuals • Despite our knowledge of physiological changes that occur during sleep, scientists do not still fully understand the functions of sleep • Many hypotheses have been developed to explain the role of sleep
Hypothesis 1: Restoration and recovery of body systems • After a period of energy consumption and breakdown (during wakefulness) the systems of the body need time to repair themselves. • Evidence against: • The brain remains active during sleep. • There is little evidence that more repair occurs during sleep than during rest.
Hypothesis 2: Energy conservation • We sleep in order to conserve energy • If this is true then total sleep time should be proportional to the amount of energy expended during wakefulness • Evidence for: • Metabolic rate is lower during sleep • Species with greater total sleep times generally have higher metabolic rates • Total sleep time decreases with age as do body and brain metabolism • Viral and bacterial infections tend to make us sleepy. This may be the body’s way to conserve energy so that the immune system may then use it to fight the infection
Hypothesis 3: Memory consolidation • Sleeping reinforces learning and memory, while at the same time helping us to forget or to clear stores of unneeded memories. • Evidence for: • People who get more REM sleep improve their ability to perform specific tasks
Hypothesis 4: Protection from predation • Evidence for: • Inactivity during sleep helps to minimize exposure to predators. • Evidence against: • Sleep decreases sensitivity to external stimuli and may, as a consequence, increase vulnerability to predation.
Hypothesis 5: Brain development • REM sleep is involved with the formation of neuronal synapses (synaptogenesis) • Evidence for: • REM sleep is markedly increased during fetal development as well as in infants (when synaptogenesis is occurring rapidly)
Hypothesis 6: Discharge of emotions • Dreaming during REM provides a release from stresses and emotions that occur during wakefulness • Evidence for: • Activity in brain regions which control emotions (amygdala), decision making and social interactions (frontal cortex) is reduced during sleep
Problems with hypotheses on the function of sleep? • No one hypothesis is ideal, they all suffer from flaws • Most fail because they cannot offer a answer for why sleep is more valuable than simply resting while remaining awake
Do all animals sleep? • Mammals, birds, and reptiles all sleep • However sleep patterns, habits, postures and places of sleep vary greatly between species • Sleep may also occur among lower life forms such as fish and invertebrates but it is hard to know because EEG patterns are not comparable to those of vertebrates
Sleep Habits • Some mammals (humans) sleep primarily at night, whereas other mammals (rats) sleep primarily during the day • Most small mammals tend to sleep longer than large ones
Unihemispheric Sleep • Some animals have unihemispheric sleep: When one side of the brain sleeps while the other side is awake • Birds do this so that they can sleep while flying South during the winter months • Aquatic mammals sleep while swimming to the surface to breathe • This allows mammals to watch out for predators at the same time as sleeping
Sleep Postures • Dogs, cats, etc, sleep curled up • Horses and birds sleep while standing • Ducks and aquatic animals sleep while swimming • Bats sleep upside down • Leopards sleep while straddling a tree • Humans sleep lying down
Sleep Places • Rabbits sleep in burrows • Lions sleep in open spaces • Hippopotami sleep underwater • Gorillas sleep in nests • Humans sleep in a bed
Journal Entry #4:Review Questions • What symptoms are experienced in individuals who are sleep deprived? • What 2 facts contradict the restorative and recovery of body systems function of sleep? • What 3 pieces of evidence support the energy conservation hypothesis? • What is the memory consolidation sleep hypothesis and what evidence supports this? • How can sleeping protect animals against predators? How can sleep make animals more vulnerable to predators? • What evidence supports the brain development and discharge of emotions hypotheses of the function of sleep? • What is one common flaw in all sleep hypotheses? • How do the characteristics of sleep (habits, postures, places) vary amongst different species?