The most restorative stage of sleep is the third stage of sleep. During this deep sleep stage, the body repairs its tissues and bones and also makes the immune system stronger. The deep sleep stage is responsible for the refreshment and repair of the physical body. It is also responsible for growth and development. This is when delta waves begin to emerge, hence this is also known as delta sleep. The muscles relax, and we drift into a deeper sleep. It lasts for approximately fifteen minutes.
As the body becomes less responsive, if a person is woken up in the third stage of sleep, they may feel out of place and disoriented for some time. Heartbeat and breathing rates are lowest during the deep sleep phase. It is during this stage that phenomena like sleepwalking, night terror, bedwetting, and sleep-talking occur. The third stage acts as a transition between the NREM stages and the REM stage of sleep and is least affected by external stimuli.