Their principal job is to eliminate excess water and waste from the circulation via urine production. The kidneys include microscopic structures called nephrons, which filter elements from the blood, return what is required to the circulation, and eliminate the remainder as urine. The remaining parts of the kidney remove toxic substances from the blood through a process called detoxication. The skin acts as an additional organ. It gets rid of waste by means of sweating and by removing toxins through perspiration, tears, saliva, and feces. Toxins are also eliminated through the lungs and immune system. Some chemicals can be broken down into smaller components that are more easily removed from the body via these other routes. For example, heavy metals can be removed from the body through the skin by washing with soap and water after exposure or taking zinc supplements if you want to get rid of the metal inside you. The brain is the last major organ to function during sleep. Because much of this activity is necessary for a healthy mind and body, most people need several hours of sleep every night. Sleep is divided into different stages. Rapid eye movement (REM) sleep is when the eyes move rapidly; it accounts for about one-third of our total sleep time. During this stage, we experience many of our dreams.