Robert M. MillerSleep is my favorite thing in life. My first waking thought is usually "when will I be able to sleep again?" But why do I love to sleep? What goes on behind my closed eyelids and what does sleep do for me? As early as 350 BC, Aristotle pondered the state of sleep: "With regard to sleep and waking, we must consider what they are: whether they are peculiar to soul or to body, or common to both." Researchers are still engaged in investigating the complexities of sleep. To answers these questions, I turned to the Internet for help. After an exhausting search, weeding through web pages designed for the average insomniac looking for a quick fix to his/her troubles, and technical research papers discussing the specifics of neurobiological functions, I found some helpful writings. What is known is that sleep is founded on patterns of bodily functions and brain activity. While we are asleep, our brains exhibit distinct and separate stages of activity. This paper will discuss the basic patterns and stages of sleep within humans and the functions of each stage. This introduction to sleep will serve as the basis for further research later in the semester.
Sleep follows distinct patterns throughout the day. Circadian rhythms are daily cycles of biological activity which mediate many of our metabolic functions. Blood pressure, heart rate, body temperature and hormonal secretions ebb and flow throughout the day in a cyclic pattern, often referred to as the "biological clock". These rhythms are controlled by the superchaismatic nucleus region of the hypothalamus, which relies on both internal timing mechanisms within specific neurons as well as outside influences to set this clock. The biological clock is kept running by "zeitbergers", that is, any input to the brain such as daylight, the smell of food, or temperature change.
Sleep cycles follow our circadian rhythms. It has been found that the natural hormone melatonin plays a large part in mediating sleep. As darkness falls, enzymes in the brain stimulate the release of melatonin from the pineal gland in the brain. Melatonin induces sleep by influencing the superchiasmatic nucleus. The release of melatonin is halted when daylight arrives, and we experience wakefulness. Other mediators such as seratonin, prolactin and prostaglandin have also been linked to the sleep cycle, but their exact role is unclear.
Upon falling asleep, the brain and body go through five stages of sleep in one sleep cycle. Researches have classified these stages of sleep by monitoring muscle tone, eye movements, and the electrical activity of the brain using an electroencephalogram (EEG). EEG readings measure brain waves and classify them according to speed. Alpha-rhythms are the fastest waves, followed by slower beta rhythms. Theta and delta waves are the slowest. A sleep cycle lasts 90 minutes or so, during which the brain revolves through each type of EEG rhythm. This sleep cycle is repeated approximately five or six times during the course of the night and corresponds to the circadian rhythms which we experience during the day.
The first four stages are considered to be Non-Rapid Eye Movement sleep (NREM) or orthodox sleep. The function of these stages is to restore and rebuild the body after a long period of wakefulness. Vegetative functions dominate NREM sleep. The body temperature, heart rate and blood pressure decrease, muscles relax and the body metabolism slows.
Stage I is a transition between sleep and wakefulness which is usually only five minutes in duration. Short dreams may occur, usually involving images remembered from throughout the day. The brain's electrical activity slows as exhibited by beta-rhythms on the EEG. Stage II is a somewhat deeper level of sleep, characterized by slower breathing and heart rates. The EEG of stage II shows slow beta-rhythms, interspersed with periods of fast alpha-rhythms called sleep spindles and some delta-rhythms. About fifty percent of all sleep in a given night is Stage II. Stages III and IV are the deepest levels of sleep and have the slowest waves as measured by EEG: Stage III has both theta and delta rhythms, while Stage IV has only delta-rhythms. The body uses this time to maintain and restore itself. Growth hormone secretions are at their highest during these stages. Stages III and IV begin after one has been asleep for approximately one half hour. The first episodes of Stage II and IV sleep are usually the longest of the night. As successive cycles of sleep pass, these stages are replaced by longer periods of Stage V sleep.
Stage V is remarkably different from the previous stages. The brain and body become active, increasing heart rate and blood pressure. The eyes shudder quickly back and forth, giving this stage the name Rapid Eye Movement (REM) sleep. EEG patterns for REM sleep are much like those during wakefulness, and include many fast beta-rhythms. It may even be that the brain works harder during REM sleep than when awake. REM sleep usually lasts anywhere from 11 to 25 minutes, typically longer in the later sleep cycles of the night. Approximately 25% of all sleep is REM sleep in adults, in children it is even higher. On completion of a phase of REM sleep, the brain and body return to Stage I, and begin another sleep cycle.
The differences between NREM and REM sleep are drastic. As mentioned above, NREM sleep deals mainly with the regeneration of the body, while REM sleep has much to do with the innerworkings of the brain. But they may certainly be interconnected. Researchers have speculated that NREM sleep also functions to recharge the brain by allowing depleted glycogen supplies to be replenished. Further hypotheses state that the high levels of neurological activity during REM sleep allows ions to be pumped across the membranes of neurons in the brain and stimulate another cycle of NREM sleep.
Researchers have found that much of the information learned throughout the day is processed by our brains during REM sleep. Other brain functions may also be mediated during REM sleep, such as problem solving, memory consolidation and creativity. An experiment performed at the Weizmann Institute in Israel tested subjects deprived of sleep. It was found that subjects deprived of sleep performed poorer on a memory recall test than subjects who had received adequate sleep. It has also been determined that rats have greater than average durations of REM sleep after being subjected to sustained sessions of learning. Information learned during these sessions can be exhibited as behavior following episodes of REM sleep. The connection between REM sleep and memory may stem from the concept that neurological synapses grow during REM sleep in adults as they do in infants. (Infants spend as much as 50% of their sleep time in REM and researchers speculate that this is when the synapses grows.)
The data processing that occurs during REM sleep may be connected to the fascinating phenomenon of dreaming. It has been speculated that dreaming allows us to organize memories into coherent patterns of learning. Although some superficial dreams occur during NREM sleep, most real dreams occur during the REM stage of the sleep cycle. There are marked differences between NREM dreams and REM dreams. NREM dreams tend to be anchored in reality and experienced as a semiconscious state of serenity. REM dreams are markedly "bizarre", lacking common sense, logic and often characterized by quick transitions in plot and setting. Often, when a person is awakened from REM sleep, they remember vividly the events of their dreams.
Dreaming during REM sleep is also accompanied by wild fluctuations in body metabolism. The heart rate, blood pressure, hormonal secretions and many other functions correspond to the events of a dream. Muscle activity, on the contrary, is almost nonexistent. There is speculation that if the body was not paralyzed during REM dreams, we may even act them out.
A great deal of research has been done to define and categorize sleep. One of the greatest tools available to scientist is the EEG. By measuring the electrical activity of the brain, stages of our sleep can be classified. Further studies on the metabolic functions of our body during NREM sleep will provide insight to how sleep refreshes and energizes. But there are still many questions left about REM sleep. There seem to be many correlations between REM sleep and learning. How do our dreams help us learn and process data? Is there a connection between REM sleep and remembering the events of the previous day? The mechanisms of these processes, as is much of the brain, are not clearly understood. But thinking about REM sleep in this respect will lead to answers in the future. Referring back to Aristotle's question, we must realize that the body does indeed sleep during the NREM stages. However the issue of whether the soul ever sleeps is still in doubt.
Aristotle. "On Sleep and Sleeplessness"
(to contribute your own observations/thoughts, write Serendip)
10/15/2005, from a Reader on the Web
THANK YOU for all the vital information on d topic of sleep by Robert M.Miller. I appreciate the fact that your site provides simple, factual information which i have greately benifitted from. THE INFORMATION PROVIDED BY MR.ROBERT IS SIMPLY PERJECT. As a student ,i found it easy to understand and use. my regards Purnima Thyagarajan
I have always been a bed wetter.I almost always wake up and feel like i haven't slept at all. Am I going into a different level of sleep? What can I do to feel rested and wake up if I need to go to the restroom? ... Andrew, 15 January 2007