During deep sleep, the body repairs itself and builds up energy for the day ahead. It plays a major role in maintaining your health, stimulating growth and development, repairing muscle and tissue and boosting your immune system. This slow wave deep sleep is essential for health, yet this stage 4 deep sleep is strongly affected by aging. Young people spend about 20% of sleep in stages of deep sleep. It is nearly absent in most people over the age of 65.