Physiology of Sleep

Stages of Sleep

       The invention of the Electroencephalogram or EEG has made it possible to monitor the brain's electrical activity. After all,neurons work through electrical impulses. The EEG is a machine that picks up and records electrical activity through electrodes attached to the scalp. The EEG of a waking person is clearly different than that of a sleeping person. The difference in EEG patterns that occur during sleep has made it possible to divide sleep into five different stages.

       The EEG pattern of our brain during wakefulness is very rapid, irregular and low on voltage. This means that many neurons are firing at different intervals and at different times, with different strengths too. It would be like going to a crowded bar and hearing people taking with each other, some here, some there; it's noisy and you can only hear a unified hum of voices without clear differentiation between them. This type of irregular electrical activity in the brain during wakefulness is called beta waves or beta activity.


       When we are relaxed, sitting in the couch watching T.V. without any mental arousal, the EEG records a waveform called alpha waves or alpha activity, slower than beta waves. These two waves (alpha and beta) represent the irregular firing pattern of large areas of the brain. Brain activity is desynchronized while in alpha and beta activity.

       Sleep is not a random process; instead, it is a very organized one. Sleep has five stages: stage 1-4 (non-REM sleep) followed by REM sleep. This process is cyclical. Once REM sleep is achieved, the cycle reverses itself and goes back though stages IV, III, II and again to III, IV and REM. Throughout the night, this cycle occurs at 90-to-100 minute intervals. Duration of sleep stages vary across ages.

Stage One

       We experience stage one of sleep when we are half asleep and half awake; our eyelids feel heavy, we feel groggy and suddenly without notice we fall asleep. Stage one counts for more or less 5% our total sleep during the night. In stage one, we are relaxed, breathing is more regular and deep. We less aware of our surroundings, we begin to slowly open and close our eyelids and begin to roll our eyes.

       Stage one is characterized by the presence of theta waves, slower and stronger waves than alpha and beta waves. Theta activity is more regular and has higher voltage than alpha and beta waves. This suggest that mental relaxation has started. We close our eyes, and within 5 to 10 minutes, we enter stage 2.

Stage Two

       In stage two, we still get theta waves plus K complexes and sleep spindles. Sleep spindles are sudden and intense bursts of electrical activity that last for a second or two. K complexes are also sudden and sharp waves of a longer frequency, often but not always triggered by noises. They naturally occur at a rate of one per minute too.
Some researchers believe that sleep spindles and K complexes help us disconnect from the outside world (noises, etc). The elderly have fewer sleep spindles and they are easy to wake up.

Stage Three and Four or Slow Wave Sleep

       After about fifteen minutes, we are ready to enter stage 3. Our bodies and minds are relaxed. The muscles of our bodies are relaxed. Our heartbeat and respiration are slow and regular.

       When we enter Stage 3, our brain activity is synchronized. This means that big groups of cells are firing at about the same time. Think about the example with people in the bar, but now instead of talking to each other randomly, they are all singing at the same. Now voices are unified; there is little noise outside the singing. This unified activity in the brain is represented in the EEG recordings by large waves called delta waves. Delta waves are the slowest and strongest waves that our brain produces. Stage 3 has about 20-50% of delta waves. Stage 4 is all delta activity. Stage 3 and 4 are also called slow wave sleep because of the appearance of delta waves.

       A young adult spends about 8% of sleep time in stage 3 and 11% in stage 4. It could be difficult to wake up somebody in slow wave sleep. However, if awakened the person seem confused and acts slow. During stage 3 and 4, growth hormone is secreted by the pineal gland. This hormone encourages bone and muscle growth in children, while in adults it is involved in tissue repair. This is another reason slow wave sleep has restorative effects. Other hormonal activities, like the secretion of prolactin and gonadotropin occur in these stages, making slow wave sleep a phase of rest and healing.

REM Sleep (Rapid Eye Movement)

       At around ninety minutes after the onset of sleep and following stage 4, the synchrony of brain waves is suddenly interrupted by desynchronized activity, similar to when we are awake (Alpha and Beta activity). At this point our muscles are very relaxed; in fact, we are paralyzed. We are mostly disconnected from the outside world, but we might wake up to something meaningful like our names being called out, or loud sounds.

       In this stage, the electrooculogram or EOG (a device that uses electrodes to pick up muscular activity of the eyes) senses that the eyes of the sleeper are moving rapidly. At the same time, the electromyogram or EMG (picks up muscle activity) senses no activity in the rest of muscles: the patient is paralyzed. The reason we might loose muscle tone is so that we do not act out our dreams, to keeps us away from trouble.

       A single REM cycle lasts about 20 to 30 minutes in a healthy person in his/her twenties. But as we will see, this period is very prolonged in newborn and children and is drastically shortened in the elder. What is common to all ages is the cycles of REM sleep-Non REM sleep.

       REM sleep occurs every ninety minute intervals after we fall asleep. This means that in an 8 hour sleep we will experience four or five REM sleep stages and every time we enter REM, we stay a bit longer in it.

       When sleepers in REM stage are awaken, usually they seem alert and can react normally. If the sleeper was awaken during slow wave sleep, it takes a few instants to wake up and react. This might be due to the presence of alpha and beta waves in REM sleep and delta activity in slow wave sleep. Remember that alpha and beta are brain waves are also present when we are awake, not only in REM. This is why some researchers believe that REM sleep is an evolutionary adaptation to wake up and be fully alert to a potential emergency in the middle of sleep.

Other characteristics of REM:

  • Blood flow to the brain is increased

  • Genital activity (in males, penis erection and in females, vaginal secretions, these not necessarily indicate that the sleeper is experiencing a dream of sexual content).

  • Dreams
  •        One of the most fascinating aspects of REM sleep is the fact that we experience dreams during this stage. Sleepers when awaken during this stage report a that they were dreaming. People woken up during stage 3 or 4 and asked what were they dreaming report they had ideas or emotions but no narrative dreamlike experiences like in REM sleep.

    How are REM and dreaming related?

           Comparing the eye movements while awake to the eye movements while in REM sleep, show almost no difference. It is thought that eye movements during REM sleep are related to the scanning of the visual scene of a dream. In other words, we are looking at the surroundings in the dream the same way we do to the surroundings when we are awake.

           Another interesting fact is that brain areas in charge of controlling and planning our body movement and areas that receive sensory information (motor and somatosensory cortex) are active during REM sleep. We have the vivid sensation of moving and feeling while dreaming. This means that we actually feel the dream as though it was real. (Go to REM and its traditional association to dreaming)

  • Glossary
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  • Physiology of sleep
  • Sleep in different age groups
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