We sleep in a regular pattern that runs about every 90 minutes. Four stages of sleep are identified that correspond with three brain wave patterns, alpha, theta, and delta. In the waking state, we spend most of our time in beta, a desynchronous state where our various waking activities keep the brain wave constantly changing. When we are more relaxed, we will exhibit more alpha waves. Alpha waves can be increased by just closing our eyes and looking upward. Meditation takes us into predominantly alpha waves. Alpha produces more synchronous brain waves.
When we first fall asleep, we move into alpha and then quickly into stage one theta. Each successive stage of sleep is associated with slower brain waves with a higher amplitude. Stage two is also theta but slower and more synchronous. Stage three is delta and stage four is also delta. Stage three delta has less than 50% delta waves and stage four has more than 50% delta waves. In the process of sleep, throughout the night, we follow a pattern of going down through the stages of sleep to stage four, which is deep sleep. In deep sleep, the body does most of its physical recovery. Respiration and heart rate slow down and body temperature decreases. We gain our deep body rest in stage four.
We go up and down through the stages of sleep throughout the night in a 90 minute pattern, as I said. In stage four sleep children experience night terrors. Night terrors, as opposed to nightmares, are usually very frightening but are usually without imagery. At least the child is seldom able to say what she or he experienced. Sleep walking may also occur in deep sleep but the sleeper will tend to move to lighter states of sleep as she or he continues to walk. Sleep talking occurs in higher delta or even stage two theta.
In a natural sleep pattern, we will spend our longest period in stage four at the beginning of the sleep cycle. This can be as long as 45 minutes. Once that cycle is finished, we begin to move upward into lighter stages of sleep until we return to stage one. This stage one is different from that we experienced briefly when falling asleep. This is dream sleep or rapid eye movement sleep (REM). It is here that we have our first dream of the night. REM sleep is also called paradoxical sleep. It is paradoxical because our brain is primarily in alpha and desynchronous beta waves. In other words, dreaming is most like the waking state as far as the brain is concerned. Also, while stage four deep sleep is characterized by the body slowing down, stage one REM sleep is characterized by just the opposite. Not only is REM sleep like a waking state, but it is like a waking state when we are in crisis. The physiological characteristics of REM or dream sleep are most like the adrenaline rush of the fight/flight syndrome. It is also called paradoxical sleep because, while it is the lightest state of sleep, it is most difficult to awaken someone from REM sleep.
During a normal REM period, regardless of the content of your dream, your heart rate and respiration are accelerated. Galvanic skin response will show an increase (perspiration). These are accompanied by the tell-tale rapid eye movements. Men will have erections, regardless of the content of the dream, and women will have vaginal lubrication. These are just some of the physiological responses during REM sleep.
While you spend up to 45 minutes in stage four sleep at the beginning of the night, you will only spend 10 minutes or so in dream sleep in the first dreaming period and then you will go back through the stages of sleep into deep sleep. This process will be repeated every 90 minutes but with some important changes. As the night progresses, you will spend less time in stage four sleep and more time in stage one dream sleep. As morning approaches, you will not go down into stage four at all. The last dreaming period of the night can last as long as 45 minutes. It is unlikely that you will have a 45 minute dream, although some of you will experience fairly long and complex dreams. During that last period of dreaming, you will likely experience three to four or more distinct dreams. These are the ones you will most likely recall in the morning, although some of you will recall earlier dreams, possibly even dreams from the first dreaming period.
What this means for the question of why people sleep is that people sleep because the deep sleep of stage four is necessary for the rejuvenation and healing of the body. When a person is deprived of sleep for a long time and then is allowed to sleep, she or he will spend more time in deep sleep than usual. The other purpose for sleep, however, is dreaming itself. It is of interest that we spend as much time dreaming, or more time, as we do in deep sleep. The body does not do these things capriciously. There is a reason for dreaming beyond the potential value of working on one's dreams psychologically or spiritually.
Dreaming also has a physiological function, one that is probably more related to the nervous system than to other bodily functions. Certainly the physiological storm of dreaming is not conducive to the recuperative relaxation of deep sleep. On the contrary (and this is now my own theory on the physiological purpose of dreaming), dreaming is like a form of natural shock therapy. The brain and nervous system are being given a real jolt, an adrenaline rush that would seem to be counter to the rejuvenation of deep sleep. The reason for the electrical storm of dreaming is hinted at in the amount of dreaming we do throughout our life. Surprisingly, new born babies dream about 50% of their sleep time, and any mother can tell you how much a new-born sleeps. Premature infants dream as much as 80% of their sleep time. When I say "dream" I obviously don't know how an infant is experiencing the dream state, but these infants are in REM sleep far more than an adult. In fact, REM sleep was first discovered in infants. An adult will spend more like 25% of sleep in REM and this decreases with aging.
Question: Why would an infant spend more time in this highly charged physiological state than an adult? Answer: Because an infant is in the process of growing its brand new nervous system. The shock of dreaming is helping the infant to grow and make new connections in its emerging nervous system. In adults, dreaming, on the physiological level, helps to make new connections or refresh frayed nerve connections brought on by the stresses of the day. Isn't it interesting that we will say that our "nerves are frayed" at the end of a hard day. I believe that something exactly like that happens to us. If we have a stressful day, our nervous system is firing like crazy as we go from one fight/flight crisis to another. Paradoxically, it is the same fight/flight adrenaline surge in dreaming that repairs these damaged nervous system connections.
We need deep sleep for our physical health. We need dream sleep for our emotional and psychological health.