Stages of Sleep (Cont'd)
As sleep progresses moving through stages 2, 3, and 4, it gets deeper and deeper, and the EEG gets progressively lower in frequency and higher in amplitude. Stage 4 consists mainly of delta activity, characterized by relatively high-amplitude waves occurring at less than 3.5 Hz. The sleeper becomes less responsive to the environment, and it becomes more difficult to awaken him. Environmental stimuli that caused him to stir during stage 1 will produce little or no reaction during stage 4. Stages 3 and 4 of sleep are also known as slow-wave sleep, which usually lasts about half an hour before the EEG begins to indicate lighter levels of sleep, back through stages 3 and 2 to the activity characteristic of stage 1. The sleeper's heartbeat becomes irregular and his respiration alternates between shallow breaths and sudden gasps. The electro-oculogram (EOG), which records electrical signals from around the eyes, shows that the subject's eyes are darting rapidly back and forth, up and down. At this point, the subject has entered another phase of sleep commonly referred to as rapid eye movement (REM) sleep.