The long held reason, and indeed function, of sleep is closely associated with the concept of body restitution. Sleep has always been perceived as a period of ultimate rest and relaxation for the body, in which tissues have a chance to repair themselves and grow, after the constant wear and tear of daytime. To start with, a basic explanation of tissue restitution is necessary. As we all know, tissues are made up cells, and these are essentially made up of protein.
Cell growth involves increase in cell number as well as size.
Both modes of growth, however, involve protein turnover, crucial process for tissue restitution. Excess amino acids are oxidized and converted into urea to be excreted in urine. Measuring protein synthesis is easier than measuring protein breakdown, which explains why only few experiments have looked at both simultaneously. An indication of the rate of protein formation is the amount of RNA present in a tissue. The various studies done on rodents have measured RNA activity, protein synthesis and mitosis, and confirmed that the peaks in these specific biological activities are mainly confined to the sleep period.