Sleep Stages

Sleep is not uniform. As we sleep our mind and body pass through different sleep stages in a somewhat regular pattern. This pattern, however, may vary considerably from night to night, evolve as we get older, and often gets disturbed by variations in our lifestyle. Based on physiological signals like EEG (electroencephalograph), eye movement and some other changes in the body, sleep can be classified into two main types or 5 sleep stages.

The 5 stages of sleep are:

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1. Non-Rapid Eye Movement (NREM) – lasting throughout about 80% of total sleep time, includes sleep stages from 1 - the initial stage to 4 – the deepest level of sleep.

The changes in the body include progressive decrease in respiratory and heart rates, blood pressure and muscle tone as follows:

Stage 1- Transition state between wake and sleep phases. Relaxing feeling letting us forget about our daily problems. EEG waves are slowly getting deeper and slower. Body temperature and heart rate decline.

Stage 2- Light sleep. There is a progression from stage 1 with EEG waves and muscle tone. Thoughts are short and fragmented. Body temperature and heart rate continue to decline. This stage takes about 50% of total sleep time.

Stages 3 and 4- are often called delta sleep, because waves in the shape of "delta" appear on the EEG waveform. This is the deepest stage of sleep, with no eye movement and no muscle activity.

2. Rapid Eye Movement (REM), or stage 5, is the time when most of our dreaming occurs. In REM sleep, muscles become temporarily paralyzed, respiration rate and depth fluctuate, heart rate is variable, temperature rises, oxygen consumption increases, EEG shows increased activity of the brain and is actually similar to awake stage, eyes move rapidly and often out of sync. There are some theories that REM sleep is associated with sorting information, memories, and learning.

The first REM of the night is relatively short, it lasts only about 5-10 minutes, than every next time increases progressively until it reaches about one hour. Adults spend about 20% of total sleep in REM sleep. In contrast, infants spend approximately half of their sleep in REM sleep. In our sleep we go through cycles between NREM and REM about every 90 minutes.

Normally there are 4 to 5 of these cycles throughout the sleep period.

External factors like drugs or alcohol may have a negative influence on certain stages of sleep, leading to sleep deprivation. Especially important is the REM phase, which plays a crucial role in psychological development, therefore deprivation of this phase may cause potentially serious mental and psychological changes. People maintaining a regular sleep schedule and getting enough sleep usually have less problems with falling asleep, feel rested and energetic throughout the day. On the other hand, those who have an irregular lifestyle and change their sleep hours from day to day may have difficulty staying focused throughout the day and falling asleep at night.

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