Scientists divide sleep into two main types: rapid eye movements (REM) or sleepy sleep, and non-REM or calm sleep. Surprisingly, they are as different from each other as waking up, but both can be important for energy.
Non-REM sleep has three phases. The last of these, known as deep sleep or slow sleep, is the primary time your body renews and repairs itself. This phase of sleep seems to be the one that plays the most important role in producing energy and improving your ability to produce ATP, the body’s energy molecule. In deep sleep, blood flow is less directed to your brain, which cools down considerably. At the start of this stage, the pituitary gland releases a pulse of growth hormone that stimulates tissue growth and muscle repair. Researchers have also found an increase in the blood levels of substances that activate your immune system, increasing the possibility that deep sleep can prepare the body to defend itself against infections.
Someone who is limited in deep sleep wakes up feeling less rested than someone who is sleeping deeply enough. When a sleep-deprived person gets a little sleep, they quickly go through the milder stages of sleep and spend more time there, suggesting that deep sleep is essential for a person’s optimal functioning.
Just as deep sleep restores your body, scientists believe REM sleep restores your mind, perhaps in part by helping to flush out irrelevant information. Various tudies of students’ ability to solve a complex puzzle with abstract shapes suggest that the brain processes information overnight.
Studies from Harvard Medical School and elsewhere have shown that REM sleep facilitates learning and memory. People who were tested to measure how well they learned a new task improved their scores after a night’s sleep. If they were prevented from having REM sleep, the improvements were lost. In contrast, improvements in scores were not affected if they were awakened from a deep sleep the same number of times.
There is also some evidence that adequate REM sleep can help maintain memory and cognitive function as we age.
reference – article from Harward Medical School.