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Physiology of Normal Sleep
Normal Sleep – Sleep Stages – Circadian Rhythm – REM Sleep – Non-REM Sleep – Clock Genes
What is Sleep?
Sleep is a special state of consciousness rather than a state of rest. During sleep the brain consolidates and sorts out the informational and emotional experience acquired during the state of wakefulness.
We sleep not because we are tired but more like we feel tired because it is time to sleep. Consequently, brain never rests – it goes through variable phases of activity and sleep is just one of those phases.
Sleep phase is essential not only for a normal sense of well-being but for survival as well. Total lack of sleep for a few days causes severe damage to multiple body functions and eventually leads to death.
Numerous biological processes go through circadian rhythms synchronized with astronomical clocks. Sleep is one of them.
Sleep is divided in two types: NREM, or non-rapid eye movements, and REM, or rapid eye movements.
Non-REM and REM are so different from each other that it is reasonable to state that there are three states of mind: wakefulness, REM sleep and Non-REM sleep.
NREM and REM run in cycles back-to-back throughout the night.
Under normal circumstances, sleep starts with stage 1 of Non-REM sleep followed by stage 2. Both stages are considered light sleep and awakening from these stages is easy.
The next stage is called “slow wave sleep”, or Non-REM, or stage 3&4. “Slow wave” term comes from the fact that recording of electrical brain activity demonstrates large low frequency waves. Awakening from this stage is normally associated with confusion and disorientation.
On the contrary to common believes, dreaming does occur during slow wave sleep. These dreams lack emotional content and consist of disorganized thoughts or random images.
Unlike in REM sleep, skeletal muscles are not paralyzed, which allows periodic tossing and turning.
Non-REM phase lasts for about 1.5 to 2 hours and is followed by REM sleep.
It is a fantastic stage! There is so much going on during REM sleep that the dull term “Rapid Eye Movements” says noting. Over centuries tons of books have been written about dreams. Believes about magical powers of prediction attributed to dreams are still alive nowadays. In reality, dreaming is not the window to the future. It’s rather a secret passage to the hidden in subconsciousness past. REM sleep is a special state of mind, which is, probably, closer to the state of wakefulness than to Non-REM sleep.
Here are some special features of REM sleep:
- Skeletal muscles are paralyzed during REM sleep. Only the eye muscles and the diaphragm (a breathing muscle) are working. Lack of such paralysis leads to REM sleep behavior disorder – dreams are physically acted out. Narcolepsy is on the opposite side of REM sleep related disorders spectrum – transient paralysis may occur while awake.
- Body temperature regulation during REM sleep is insufficient. There is no sweating and no shivering. Animals, subjected to chronic REM sleep deprivation, develop severe temperature control problems.
- Dreams are playing an important role in memory consolidation and settling of emotional conflicts. The percentage of REM sleep is increased in depression.
- Males experience erection during REM sleep. This phenomenon finds its use in the evaluation of erectile dysfunction.
REM phase usually lasts from 30 to 40 minutes. REM sleep phase is longer towards the morning hours. After REM sleep is over the cycle repeats and stage 1 of Non-REM sleep follows.
An adult 24 hour sleep-wake cycle does not start right after birth. It gradually forms over years. At birth, sleep pattern is irregular with total sleep hours ranging from 16 to 18 per day. This pattern gradually transforms into longer night sleep with shorter naps and consolidates into a single nocturnal sleep by the age of 6 years.
In adults, an average sleep time at night is about 7.5 to 8 hours. Individual sleep requirements are genetically predetermined, so normal sleep time has a wide range between 4 and 10 hours per night.
Teenagers and adolescents have a tendency of going to bed late, while requiring longer sleep time than adults. It’s the reason for relatively common delayed sleep phase syndrome in these age groups. In contrast, older people tend to wake up earlier.
Experiments show that internal clocks are set for about 25 hours per cycle. External cues, such as daylight and social activities, are responsible for synchronization of internal and astronomical clocks.
Sleep structure changes over years. Fetus has almost exclusively REM sleep. REM sleep percentage is gradually diminishing from 50% at birth to 20-25% by 4 years of age. Infants also have a very large percentage of deep sleep, which goes down to 25% in adolescence. The percentage of deep sleep gradually diminishes over years. The elderly have minimal amount of deep sleep stages.
Light is an important factor in maintaining sleep-wake cycle. Melatonin does not appear to be critical in human sleep-wake cycle besides some influence on the pattern of circadian body temperature variation.
Other Circadian Rhythms
Sleep-wake is the most obvious cycle but there are other functions that follow a similar pattern:
- Blood pressure is the lowest at 3 am.
- Skin cells divide most actively around midnight.
- Heart attacks are most common at 7 am,
- Asthma attacks and strokes tend to cluster around 4 am,
- Brain hemorrhage occurrence is the highest at 8 pm,
- Blood pressure peaks at 9 pm.
- The worst time for surgery is 1 am.
- The highest death rate for any reason is at 4 am and the lowest at 6 pm.
- The best mental performance is at 3 pm.
- The body temperature is changing throughout the day and depends of the sleep stage.
- There are even special “clock” genes that regulate multiple functions in the body around the clock. This explains why some of circadian rhythm abnormalities are inherited.
Regulation of Sleep-Wake Cycle
There are special brain areas that work like switches between different states of consciousness: awake, Non-REM sleep and REM sleep. Multiple feedback loops tightly regulate the system. Staying awake is an active process. A special reticular activating system is responsible for maintaining the state of wakefulness. Loss of this activation system due to brain trauma or encephalitis may lead to a coma. A small lesion in the brainstem may shut down the whole brain causing a permanent sleep-like state.
Sleep-wake system is set in such a way that under normal conditions NREM has always given the priority. It is very important to keep REM sleep far away from wakefulness. In REM sleep, muscles are paralyzed and dreaming runs nonstop. It’s not hard to imagine what will happen, if drowsiness starts with REM sleep while driving. Is it possible to drive while being paralyzed and while hallucinating!?
In narcolepsy, control over REM sleep is lost. A narcolepsy sufferer may suddenly loose muscle tone and fall down while being awake.
Hyperactive activation system generates problems of its own. People with chronic insomnia, in spite of hours of sleep, may feel that they have not slept the whole night. It happens because the brain is running both, awake and sleep states at the same time.