The alternating periods of Slow-Wave Sleep (SWS) and Rapid-Wave Sleep (REM) are the ones that characterize a typical sleep. Generally, a person starts sleeping in the first stage of SWS. It is difficult to differentiate the EEG of a person in this stage from that of a person who is waking up. There is a decrease in the rate of heart and muscle tension. People who are in this stage are in most cases unaware that they are indeed sleeping. In this stage of sleep, there might be marginal disturbances which could sometimes be followed by jerking of a muscle.
This jerking of the muscle is referred to as myceclonia. This might occur in the arm or leg. The second stage of SWS is reached after a period of ten to fifteen minutes. At this time, the person is definitely asleep. The person experiences a further decrease in the rate of the heartbeat as well as muscle tension. Shorts bursts of sleep spindles are experienced on the EEG.
These bursts occur in 12 to 14 cycles per second. In the second stage, there is the experiencing of unique short bursts of activity referred to as the K-complex. The third and fourth stages of sleep are reached after approximately fourteen minutes in the second stage. In these two stages, delta wave activity is observed. The period of SWS goes on for about ninety minutes after which REM sleep in its first period starts to occur. Paradoxical sleep is also witnessed during this stage. This is a reflection of the brain activity taking place in this stage that is similar to being awake with the outside appearance of a person sleeping deeply. Occurrences during this stage include having vivid dreams.