Stage I sleep is the first phase of the sleep cycle, transitioning from wakefulness into the early stages of deeper sleep. It is a relatively light stage of sleep, typically lasting only a few minutes, and is characterized by changes in both physical and brain activity. Here's a more detailed explanation of the key features:
Drowsiness and Eye Movements
As you begin to fall asleep, your body and brain gradually relax. This leads to a decrease in the frequency of eye blinks and slower, more deliberate eye movements, often referred to as "roving eye movements". During this phase, you might experience the sensation of "floating" or jerking movements as your muscles relax, a phenomenon called hypnic jerks.
Diffuse Attenuation of the EEG Tracing
The brain’s electrical activity, measured through an EEG (electroencephalogram), shows a noticeable change. The Posterior Dominant Rhythm (PDR), which is a pattern of rhythmic brain waves seen when awake and relaxed (usually in the alpha frequency range, 8-13 Hz), gradually diminishes as you fall asleep. As sleep deepens, the EEG shows a more diffuse and less organized pattern, indicating the transition from wakefulness to sleep.
Positive Occipital Sharp Transients of Sleep (POSTS)
POSTS are sharp, positive waves that appear in the occipital region of the brain (the back of the head). They resemble spikes but are completely benign (not related to epilepsy). These waves have a "sail-like" appearance, which can be seen either as single events or in short bursts (runs). While POSTS are specific to stage I sleep, they can persist into later stages of sleep (particularly stage II) and are considered a normal part of the sleep process. It's worth noting that POSTS have a counterpart in the awake state, called lambda waves. Lambda waves look similar but are usually observed when a person is visually processing stimuli, such as focusing on an object.
Vertex Waves
Vertex waves are bilateral, phase-reversing electrical discharges that occur over the central regions of the brain. They are typically seen as sharp waves or complexes. Even though vertex waves may have a sharp appearance and can sometimes seem alarming, they are a normal feature of Stage I sleep. These waves can occur as isolated events or in bursts. They are not indicative of epilepsy or any other pathology.
In children, vertex waves are especially common and may appear more intense or frequent, but they are still considered normal and non-pathological.
Importance of Stage I Sleep
Stage I sleep is crucial for the initiation of the sleep cycle. It's a transitionary phase where the body begins to relax, heart rate and respiration slow, and brain activity shifts toward sleep. This phase sets the foundation for the deeper stages of sleep that follow, such as Stage II and slow-wave sleep (Stages III and IV), which are important for restorative functions like memory consolidation, physical repair, and overall health.
Summary
In summary, Stage I sleep is a light, transitional phase marked by specific changes in EEG patterns, including the appearance of POSTS and vertex waves. These changes are entirely normal and signal the shift from wakefulness to the deeper stages of sleep.