The body never truly rests. Even if we are unaware of it, our nervous system is active during this time. In fact, our brain is responsible for monitoring the entire sleep cycle. In particular, we dream during the REM cycle. We may not always remember them, but we always dream. During the REM cycle, our eyes move. In fact, the REM cycle is named after our eye movements. REM stands for rapid eye movement. The eyes are part of our sensory system.

We never stop breathing when we rest, so our respiratory system continues to function. At the most, our breathing will become irregular or slowed. Our blood (part of the circulatory system) also continues to circulate throughout the body. During stages 3 and 4 of the sleep cycle, our blood pressure will drop.

Most people will toss and turn in their sleep so both the muscular and skeletal systems continue to function. In particular, blood circulation to the muscles will increase during the 3rd and 4th stages. It is worth noting that your body becomes paralyzed during the REM cycle. This is the one case where a body system will actually rest.

Our immune system is at its best when we sleep. This is one of the reasons why plenty of rest is recommended when fighting infection. During sleep, the production of disease-fighting agents such as TNF increase. Many hormones from the endocrine system are also secreted during sleep such as the human growth hormone, melatonin, testosterone, and follicle stimulating hormone.