<P> J.A. Horne (1978) reviewed several experiments with humans and concluded that sleep deprivation has no effects on people's physiological stress response or ability to perform physical exercise . It did, however, have an effect on cognitive functions . Some people reported distorted perceptions or hallucinations and lack of concentration on mental tasks . Thus, the major role of sleep does not appear to be rest for the body, but rest for the brain . </P> <P> When sleep - deprived humans sleep normally again, the recovery percentage for each stage of sleep is not the same . Only seven percent of stages one and two are regained, but 68 percent of stage - four slow - wave sleep and 53 percent of REM sleep are regained . This suggests that stage - four sleep (known today as the deepest part of stage - three sleep) is more important than the other stages . </P> <P> During slow - wave sleep, there is a significant decline in cerebral metabolic rate and cerebral blood flow . The activity falls to about 75 percent of the normal wakefulness level . The regions of the brain that are most active when awake have the highest level of delta waves during slow - wave sleep . This indicates rest is geographical . The "shutting down" of the brain accounts for the grogginess and confusion if someone is awakened during deep sleep since it takes the cerebral cortex time to resume its normal functions . </P> <P> According to J. Siegel (2005), sleep deprivation results in the build - up of free radicals and superoxides in the brain . Free radicals are oxidizing agents that have one unpaired electron, making them highly reactive . These free radicals interact with electrons of biomolecules and damage cells . In slow - wave sleep, the decreased rate of metabolism reduces the creation of oxygen byproducts, thereby allowing the existing radical species to clear . This is a means of preventing damage to the brain . </P>

Which of the following stages of sleep is also known as delta sleep