If you wanted to describe sleep, it is not so difficult to say some things that you know from experience but what actually is happening while we sleep and dream? First of all answering this question I should say that sleeping is not an easy procedure for our brain. Some areas of our brain are active while asleep and some others while we are awake. When we wake up histamine* is released and enables other neurotransmitters* to take action. Thalamus which is the region that is responsible for the appropriate function of all our senses except hearing is finally active. That means that we are capable of seeing, touching, smelling and sniffing. Adenosine builds up when we are awake and when it reaches high levels, it is a sign that we need sleep. While we are asleep a neurotransmitter is responsible to inhibit the function of all the active areas.
Sleep is separated in two types nREM (non Rapid Eye Movements) and REM (Rapid Eye Movement) and according to the literature the average human adult enters sleep through nREM and after that nREM and REM alternate through the night with an approximately 90 minute cycle. The first one has 4 stages. According to the first stage (transition from wake to sleep) tends to produce a memory impairment and this is why we can never recall when we fall asleep. This stage is easily disrupted and the average of duration is 5 minutes. The duration of the following stage (2) can be from 10 to 25 minutes and after that stage three has a very limited duration and is being used as a transition to the last stage. this lasts approximately 20-40 minutes in the first cycle and is our deepest stage of sleep. While nREM we have no dreams and our brain is inhibited in order not to be responsive to external stimuli. When REM type is coming we are starting to watch dreams. Everybody watches dreams but some people have difficulty recalling them the next morning.
Watching dreams is a complicated activity that is happening and the most amazing thing that you need to know is that our body is deliberately paralysed in order not to start moving. How is this happening? While we are in the nREM type everything is inhibited but when we enter REM, thalamus is starting to communicate with our cortex. Our cortex has a lot of functions such as memories, movements, language production ect. As a result with this activation all of those functions are starting to power and we dream but in order to stay in bed, and not start to move, our brain sends neurons in the spinal cord which is shutting off all muscle activity. These neurons are literally paralysing our whole body leaving only our brain to think and dream. This is called REM atony. For example, when we dream that we are playing a football game and we are shouting and moving we will actually have vivid images of that but not the movements. This is amazing because it proves that our brain is also quit active when we are asleep but we are not aware of that. People that are sleepwalking have abnormalities in this system.
Sleep is an amazing function and it proves that our brain is always active and never rests while our body often asks for some rest. This is another reason why we should respect our brain as it is always on the run.
*histamine = neurotransmitter that promotes wakefulness
*neurotransmitters = chemical compounds that transmit messages