RAPID EYE MOVEMENT SLEEP DISORDER

Rapid eye movement sleep is recurring sleep state during which dreaming occurs.
Sleep is a periodic state of rest during which consciousness of the world is suspended.
The most significant characteristic of sleep, which differentiates it from the waking state, is the interruption of perception; a sleeping person does not see or hear.
Additionally, sleep is marked by:

• Slowed-down metabolism,
• Active and complex brain wave patterns and
• Decreased movement of the skeletal muscles.

What are the stages of sleep?

Normal sleep has 2 distinct states: non–rapid eye movement (NREM) and rapid eye movement sleep (REM sleep). There are five stages of sleep: stages 1, 2, 3, 4 and Rapid eye movement (REM sleep).
NREM sleep is divided into 4 stages. These stages progress in a cycle from stage 1 to Rapid eye movement sleep, then the cycle starts over again with stage 1. Each stage represents a different physical and mental state of the body during sleep.
During some stages, the body is in a lighter sleep and can be awakened more easily, while others indicate a very deep sleep. We spend almost 50 percent of our total sleep time in stage 2 sleep, about 20 percent in Rapid eye movement sleep, and the remaining 30 percent in the other stages. Infants, by contrast, spend about half of their sleep time in Rapid eye movement sleep.
The descriptions of five sleep stages are:

• Stage 1 (Light Sleep) – During the first stage of sleep, we're half awake and half asleep. We drift in and out of sleep for about 5 to 10 minutes and can be awakened easily. Our eyes move very slowly and muscle activity slows. People awakened from stage 1 sleep often remember fragmented visual images.
• Stage 2 (True Sleep) – Our eye movement stops and our brain waves (fluctuations of electrical activity that can be measured by electrodes) become slower, with occasional bursts of rapid waves called sleep spindles. The breathing pattern and heart rate start to slow down.
• Stages 3 and 4 (Deep Sleep) – Slow brain waves called delta waves, a type of wave that is large (high amplitude) and slow (low frequency) begin to appear, interspersed with smaller, faster waves. Rhythmic breathing and limited muscle activity characterize stage four. By Stage 4 the brain produces delta waves almost exclusively.
  
  It is very difficult to wake someone during stages 3 and 4, which together are called deep sleep. There is no eye movement or muscle activity. People awakened during deep sleep do not adjust immediately and often feel groggy and disoriented for several minutes after they wake up. Some children experience bedwetting, night terrors, or sleepwalking during these stages.
• Rapid Eye Movement – The first rapid eye movement (REM) sleep period usually begins about 70 to 90 minutes after we fall asleep. We have around three to five Rapid eye movement sleep episodes a night. During Rapid eye movement sleep, our breathing becomes more rapid, irregular, and shallow, our eyes jerk rapidly in various directions, and our limb muscles become temporarily paralyzed.
  
  Our heart rate increases, our blood pressure rises, and males develop penile erections. People dream during this stage. Although we are not conscious, the brain is very active - often more so than when we are awake. This is the period when most dreams occur. Our eyes dart around (hence the name), breathing rate and blood pressure rise. However, our bodies are effectively paralysed, said to be nature's way of preventing us from acting out our dreams. After REM sleep, the whole cycle begins again.

The first cycles of the night will tend to have shorter Rapid eye movement sleep periods and longer periods of deep sleep. This trend reverses as the night goes on. The later cycles have longer Rapid eye movement sleep periods and shorter deep sleep periods.
Infants are unique in that they spend approximately 50 percent of their sleep time in Rapid eye movement sleep. By morning, most sleepers spend almost all of their time in stages 1, 2 and Rapid eye movement sleeps with very little or no deep sleep (stages 3 and 4).
The average length of time for a complete sleep cycle is 90-110 minutes. About 50 percent of sleep time is spent in stage 2 and about 20 percent in Rapid eye movement sleep. The remaining 30 percent is split among the other stages.
On average, a person will cycle through the stages 4 or 5 times in an eight-hour period. After a person falls asleep, the first Rapid eye movement sleep period generally happens 70-90 minutes later.