Biological cycles within our bodies that occur on an approximate 24 hour cycle are known as circadian rhythms. Circadian rhythms promote a readiness for sleep by decreasing alertness but they do not regulate sleep. Instead about every 90 minutes while asleep our body cycles through different stages in which brain activity and other physiological responses change in a predictable way.
Scientist study sleep by monitoring the brain wave activity of people while they are asleep. This is how they have determined the different stages of sleep and when a person is in each level. It works something like this, when a person is a wake and conscious, their brain will produce a pattern of beta waves. These waves are a high frequency but a low amplitude or height. As you close your eyes and get that drowsy, relaxed feeling, your brain wave activity will start to slow down and alpha waves will start to occur.
As sleep starts, the brain wave pattern becomes more irregular and slower theta waves increase. This is known as stage 1 sleep. This is a form of light sleep from which a person could be woke up rather easily. This stage normally lasts a very short time and some people do experience dreams in this state. As you fall deeper into sleep,1-2 second bursts of rapid brain wave activity, known as sleep spindles, begin to occur. This is an indication that a person is now in stage 2. The muscles are more relaxed, breathing and heart rate slow down and dreams may occur. It is harder to wake someone in this state up than in stage 1.
Sleep deepens further as you go into stage 3. This stage is noticed by a regular appearance of very slow and large delta waves. As time goes by they occur more often and when they dominate the pattern of the EEG you have reached stage 4. Your body is relaxed, and brain activity over a large portion of the brain has decreased. After about 20 minutes in stage 4 the EEG shows that you go back through stage 3 and stage 2 spending just a small amount of time in each. In about 90 minutes you will have completed a cycle of the stages going: 1-2-3-4-3-2. After this time you will enter a different stage of sleep.
This stage of sleep is known as REM sleep. Or rapid eye movement. This stage of sleep is characterized by the rapid eye movements, high arousal, and frequent dreaming. It has been found that anyone woken up during this state report dreams. Even if it is someone who says they never dream.
During REM sleep, physiological arousal may increase to daytime levels. The heart-rate quickens, breathing becomes more rapid and irregular, and brain wave activity even resembles that of being awake. During this stage the brain also sends signals making it more difficult for voluntary muscles to contract. As a result, muscles in the arms, legs, and torso, lose tone and become relaxed. The muscles may twitch but in a since you would be paralyzed. In this state your body is actually highly aroused but since there is no movement a person looks as if they are sleeping peacefully.
Although it takes about 90 minutes to cycle through the sleep stages research has shown that as time passes while a person is sleep stages 4 and 3 eventually drop out and REM sleep periods become longer.
We need sleep in order to replenish and recharge our bodies in an effort to recover from mental fatigue and physical fatigue. Exactly what it is that needs to be restored in our body is a mystery; however, most scientists believe we need sleep to rid our body of "adenosine". This is a byproduct that cells give off when they consume food. Scientists believe that as adenosine accumulates in our bodies it stops the brain from sending signals to keep us awake therefore signaling the body to shut down.
Adenosine levels decrease as we sleep. Interestingly though, caffeine has a molecular structure similar to that of adenosine. Caffeine fits into the adenosine receptor sites but doesn't stimulate them. This blocks the actions of the adenosine and prevents the adenosine from signaling the brain to slow down thereby increasing alertness. So now we have found how caffeine may keep us awake.