During the day when you are awake, the conscious mind does 90% of the work. This is where the major part of hertz brain waves are seen. Beta is the normal level for individuals who are alert and conscious with their eyes open during the day. Beta activity is fast. It is dominated by frontal lobe activity.

When day is done as we relax and prepare to sleep, the brain drifts into the Alpha state. We all experience the alpha state twice a day, naturally, when we fall asleep and awaken. The Alpha state is where hypnosis occurs. If this is a state of mind that we all experience every day, naturally, it is not to be feared or concerned about.
Alpha is usually best seen in the posterior regions of the brain on each side, known as the cerebral cortex. It is brought out by closing the eyes and by relaxation, and ends with sensory input and rational thought. As brain waves slow during the Alpha level the states of conscious move from the meditative to the REM (REM means a state where Rapid Eye Movement occurs) state and on to a light hypnotic state.



Alpha turns to theta as the state deepens. This is the learning state of mind as used by those who have tried sleep learning.( during this time the brain reacts just as it does during wakeful hours where learning has occurred only -without sensory output. Theta activity has a frequency of 3.5 to 7.5 Hz and is classed as "slow" activity. As sleep gets deeper brain activity begins to slow. At the earliest of the Theta levels the mind is at its most creative point and the most revealing dreams will occur at the beginning of the Theta wave state.
Delta activity is 3 Hz or below. It tends to be the highest in amplitude and the slowest waves. It dominates stages 3 and 4 of the sleep cycle.

The final stage is our deepest level of sleep. Brain activity slows and the body releases chemical and hormones to repair tissue and spur on growth. Hormones such as seratonin and human growth hormone are released during phase four. Bodies undergoing changes or stress normally require a longer time in stage four sleep. Teenagers typically spend more time in stage four as their bodies require more growth hormone activity. As we age, stage four sleep generally shortens and may be eliminated altogether by age 60 or 70. This explains why sleeping time generally shortens as we age.
To put this simply, we progress through the stages of brain activity throughout the night, (between Alpha, Theta, and Delta) The sleep states of the brain fluctuate three or four times a night, depending on the individual, their age, and so on, so you may go through varying stages of brain activity which will vary the depth of sleep during the night.