All humans sleep, and it is necessary for everyone to receive an adequate amount of sleep each night. Lack of sleep affects your overall health and mental status. For many decades, scientists have studied sleep, and research has proven that humans need sleep in order to remain focused and energetic. Sleep affects the human brain in so many ways. It is necessary for psychologists, researchers and scientists to thoroughly understand the connection between sleep and the brain. This paper will take a look at the sleep cycle. It will focus on the following aspects of sleep: benefits of sleep, stages of sleep, how the brain functions during sleep, dreams, measurements, circadian rhythms and sleep deprivation. I will also provide suggestions for acquiring a good night’s sleep. Overall, this paper will reflect on the significance of sleep and how it affects the body.
Benefits of Sleep
I am sure that everyone knows the importance of receiving a good night’s sleep. I wonder how many people actually know how essential sleep is to our health. It is necessary for everyone to sleep. But why do humans need to sleep? Naturally, we need a good night’s sleep in order to stay awake and remain focused (Siegel, 2003). One cannot properly function on a daily schedule without an appropriate amount of sleep. However, there are other benefits for obtaining an adequate amount of sleep.
According to Ravishankar (2006), sleep is a key factor in maintaining good health. Sleep is a necessity, just like food, air and water (Ravishankar, 2006). Acquiring an adequate amount of sleep will result in you feeling alert and refreshed. When your body lacks sleep, you may feel confused and fatigued. Also, lack of sleep has a negative impact on your social and mental ability (Ravishankar, 2006).
There are also additional benefits of receiving a good night’s sleep. Ravishankar (2006), states that sleep aids your immune system in fighting off bacteria and viruses. Also, sleep allows your body to repair and clean out bad elements found in the body. In addition, research studies show that sleep rejuvenates the nervous system and brain, and regulates blood circulation (Ravishankar, 2006).
In 1953, Nathaniel Kleitman, a sleep research pioneer, and his student Eugene Aserinsky, stated that sleep was more than the brain ceasing from most activity. Through research, the these two individuals discovered that sleep was identified by occurrences of “rapid eye movement”. They also discovered that something active happened while we slept (Siegel, 2003).
Can you imagine staying awake for days at a time? Imagine how it would affect your schedule. There have been a few times when I have stayed up all night. The next day I was drowsy and irritable and I was ready to jump into my bed and fall asleep. My brain could not function properly. I soon realized how important a good night’s sleep meant to my health.
The amount of sleep each individual needs varies (McClearly, 2006). Everyone’s body does not operate the same, therefore the required amount of sleep will not be the same for everyone. You will know when you have received enough sleep because you will feel renewed and attentive, and you should be able to think and complete tasks with no problem.
Izac (2004), states that lack of sleep has a direct impact on your ability to think clearly. Certain research studies suggests that sleep can affect learning and memory (Ravishankar, 2006). If you do not sleep, your brain will become tired and you will lose concentration on your daily activities (Ravishankar, 2006). Therefore, everyone should strive to get a good night’s sleep on a regular basis.
Adequate sleep, just like exercise and proper diet, is necessary for good health (Ravishankar, 2006). There are so many benefits for getting a good night’s sleep. It makes me wonder why so many people deprive themselves of it. When you sleep, your body can rest, relax and recuperate. I feel that it is mandatory to allow your body the time to sleep and prepare for the next day. Getting an adequate amount of sleep each night is a priority for me. I always try very hard to get a good night’s sleep.
Stages of Sleep
There are two categories of sleep: NREM (Non Rapid Eye Movement) and REM (Rapid Eye Movement). The average adult in good health, usually falls asleep within ten minutes each night, and travels through five distinct stages. NREM consists of four stages and REM consists of one stage (Izac, 2004). During the night, a person continuously goes from one stage to another. These cycles are known to last anywhere from 70 to 90 minutes (Ravishankar, 2006). According to Ravishankar (2006), early REM periods are usually 5 to 10 minutes in length. However, a person may experience several extended REM periods throughout the night.
According to Ravishankar (2006), different activities occur in each of the five stages of sleep.
1. Stage 1- Light Sleep – We drift in and out of sleep
2. Stage 2- True Sleep – Eye movements stop and brain waves become slower
3. Stage 3- Deep Sleep- Delta waves are produced (slow wave sleep)
4. Stage 4 – Deep Sleep – Rhythmic breathing and limited muscle activity
5. Stage 5- Breathing becomes more rapid, irregular and shallow; dreams occur
During sleep, the brain’s activity drops by 40 percent. However, the brain remains active during the first phase of sleep (Ravishankar, 2006). In REM sleep, we breathe erratically, with apneas occurring from 10 to 30 seconds in length (Izac, 2004). REM sleep is identified by “high brain metabolic and neuronal activity rates, reduced muscle tone, irregular and relatively automatic respiration uncoupled from its usual regulatory mechanisms” (Siegel, 2005).
In NREM, we breathe in a regular pattern, with a more detailed hypoventiltion (Izac, 2004). The NREM-REM sleep cycle occurs four to six times throughout the night; depending on the total amount of sleep acquired (Izac, 2004).
How the Brain Functions During Sleep
Research shows that the human brain is definitely impacted by sleep. While we are asleep, our brains are still busy at work. It is amazing how the brain is still completing tasks even while we are asleep. There is a neuronal system that controls the periods of sleep and wake and this system is located in the isodendritic core. It stretches from the medulla, throughout the brainstem and hypothalamus and finally reaching up into the basal forebrain (Izac, 2004). This core contains different neurons, which are important for wake and sleep. Both the sleep-active and wake-active neurons are intertwined (Izac, 2004).
Specific particles within the “cerebral spinal fluid, insulin, gut hormones, cytokines, and other such peripheral factors facilitate sleep and influence the sleep-wave cycle” (Izac, 2004). According to Izac (2004), the following areas have an abundance of sleep-inducing neurons: “basal forebrain, preoptic area, lower brainstem reticular formation and the rostra hypothalamus”.
Slow wave sleep has high EEG rhythms, which activates a large amount of neurons. These EEG waves are inactive during the wake and REM stages. The GABA sensitive neurons are found in the thalamic nucleus and are believed to be associated with slow wave sleep. Slow wave sleep is known for having a decrease in cellular firing in most areas of the brain, “compared to waking levels.” Slow wave sleep causes a reduction in activity in the thalamocortical system, with an increased amount of activity in the certain areas of the limbic system (Izac, 2004).
The sleep cycle is regulated by two pathways consisting of effector neurons. These neurons are designed to suppress or promote muscle atonia, rapid eye movements or EEG desynchronization found in REM sleep (Izac, 2004). Certain mechanisms found in the forebrain, may enhance REM sleep (Izac, 2004). According to Izac (2004), research has shown that monoaminergic neurons (containing norepinephrine and serotonin) may actually suppress REM sleep. In addition, research suggests that cholinergic neurons (containing acetylcholine) may possibly promote REM sleep (Izac, 2004).
It is possible that the lateral pontine area of the brainstem, serves as the “command and control center for REM sleep” (Izac, 2004). REM sleep-on and REM sleep-off cells, produce cellular activity during the REM stage (Izac, 2004). REM sleep also produces PGO waves. These waves are spiky EEG waves that appear in the pons. From the pons, the PGO waves are sent to the lateral geniculate nucleus, and then to the visual occipital cortex. The PGO waves are also found in the nonvisual thalamic nucleus. Here, the PGO waves timing is specifically connected to eye movements (Izac, 2004).
According to Izac (2004), most brain areas display increased activity during sleep, when compared to slow-wave-sleep. REM sleep is very similar when compared to waking sleep. However, the limbic system is not active during REM sleep. During REM sleep, more individual neurons are fired throughout the brain’s motor systems. There is an increased amount of activity occurring in the cerebellum, cortex, and the pyramidal tract. Nevertheless, a majority of the motor neurons are “inhibited during REM” (Izac, 2004).
Sleep and Dreams
Dreams occur in the REM stage of sleep. For many years, researchers have studied dreams and their association with sleep. Dreams are an important component of the sleep cycle. Freud and Adler made great contributions to the area of dream interpretation and analysis. According to Bird (2005), dreams can be beneficial because they enable individuals to understand their lives and the options available to them.
The first known book about dreams is entitle “Oneirocritica”, composed by Artemidorus (Bird, 2005). During that time, the Greeks categorized dreams into two groups. One group consisted of dreams that were influenced by the present or the past, but had no importance. The other group consisted of dreams that involved the future (Bird, 2005).
Solomon Almoli, is known for writing “The Interpretation of Dreams,” during the 1500’s. Almoli, found dreaming to be important for everyone. He viewed dreaming to be a gift and blessing to those that understood the messages found in dreams. These messages helped individuals to navigate their personal life (Bird, 2005).
Over the past 40 years, scientists have provided some helpful information in regards to dreams. Dreams assist in repairing the “wear and tear on the cerebrum” (Bird, 2005). Also, dream episodes are most likely to happen four to six times each night (Bird, 2005). According to Bird (2005), each person probably dreams for approximately 2 hours in a night but everyone will not remember his or her dreams. Even healthy people may not recall their dreams. So, if you can not remember your dreams, you do not have to fear. I personally remember my dreams practically every night. I find it exciting and enlightening to recall my dreams. Bird (2005), states that when people have a desire to remember their dreams, they are more capable of doing so.
Bird (2005), explains that when individuals remember their dreams, they usually do not understand them. Adler, focused on discovering the purpose of dreams and the outcome. He felt that the overall purpose of dreams was to create a mood or an emotion (Bird, 2005).
Freud stated that there were two purposes of the dream: “the production of the dream thoughts and their transformation into the content of the dream.” Freud viewed dream thoughts as being rational (Bird, 2005). Freud also believed that we transform our conscious thoughts and desires into our dreams. There are still some mysteries involving the dream world. However, research has shown that everyone does dream, and some dreams have significant meaning to individuals.
According to Porte (2004), there are several methods available to measure human sleep. The four measurements listed below can provide very helpful information about a person’s sleep pattern.
1. Electroencephalpgram (EEG) – Records brain wave activity
2. Electrooculogram (EOG) – Traces eye movements
3. Electromyogram (EMG)- Measures muscle tension
4. Polysomnogram (PSG)- Records vital signs and physiology during a night of sleep
The above instruments have been used in many research studies. Porte (2004), discusses one study in particular. Porte (2004), discusses a research study that focuses on “slow eye movement at human sleep onset”. SEM (slow eye movement) at the onset of sleep, is labeled as “drifting” or “rolling” (Porte, 2004).
In this study, the subjects were five college students, in good health, that didn’t smoke. There were three females and two males, with the average age of 20.8 years. They were paid volunteers. It was estimated that each study participant would have 40-50 usable SEM’s, producing a sample of approximately 200. The subjects were required to spend two nights in a laboratory. The participants could not consume caffeine or alcohol or take a nap within 24 hours of sleep recording. In addition, no subject was currently taking any medications (Porte, 2004).
The EEG and EOG instruments were used in this study. The subjects’ eye movements were measured by the EOG, and the brain wave activity was recorded by the EEG. The five subjects contributed 43, 45, 57, 56, and 37 SEM’s, respectively (Porte, 2004). This resulted in a total sample of 238. The duration and peak velocity “produced main sequences as functions of SEM amplitude” (Porte, 2004). Measurement of human sleep is essential to sleep research. So much valuable information has been discovered by using the instruments discussed in this section.
Sleep Cicardian Rhythms
The suprachiasmatic nucleus (SCN), is classified as the brain’s biological clock (Someran, 2000). The SCN is made up of small masses of nerve cells located within the ventral hypothalamus (Izac, 2004). The SCN is responsible for the sleep and wake activity in the brain.
The SCN receives messages from the retina, through the retinohypothalamic tract. This allows the circadian sleep-waking rhythm to be programmed by the daylight-nighttime cycle of our environment (Izac, 2004). The SCN extends to a large area of the brain, including the brain stem, pituitary, pineal gland, and other regions of the hypothalamus.
Many humans are programmed to the 24-hour day and they get acquainted with a normal sleep-wake pattern (Izac, 2004). However, changes to our body’s circadian rhythm can lead to sleep disturbances (Someran, 2000).
According to Someran (2000), there is a high number of sleep disturbances in the elderly. Studies show that 40 to 70% of elderly people are victims of chronic sleep disturbances. Elderly people suffering from sleep disturbances normally have frequent awakening during the night, trouble falling asleep, and/or they wake up too early (Someren, 2000). Sleep disturbances have also been associated with illnesses including hypertension, cardiac insufficiency and angina pectoris (Someren, 2000).
When the SCN ages, there is a decrease in neuronal activity (Someren, 2000). Research studies using aged rats have proven that sleep-wake patterns and the SCN’s cell function can be restored by “enhancing the stimuli the circadian timing system normally uses for synchronization of the sleep-wake rhythm.” Research suggests that the same process may be helpful in humans (Someren, 2000).
For those individuals that are currently on a normal sleep-wake cycle, the body’s temperature is lowest at night. Also, the melatonin levels are highest when we are asleep at night (Izac, 2004). A person’s cortisol level is low when he or she first falls asleep. However, the cortisol level is high when you awaken in the morning (Izac, 2004).
It is important for everyone to adapt to a regular sleep-wake cycle. However, certain lifestyle changes can alter your sleeping pattern. For example, having a new baby in the home, changes in your work schedule, traveling and changing time zones, etc. I can remember working on a previous job which required me to stay up all night. For example, my work schedule was from 6:00 p.m. until 6:00 a.m. That was a major change for me and I had to program my mind and body to sleep during the day, and stay up all night to work. I can honestly say that I prefer working days or evenings compared to working overnight. Nevertheless, it is necessary to adjust to any changes in your schedule. You must still receive an appropriate amount of sleep in order to remain healthy.
How Much Sleep Do We Need?
So, how much sleep does everyone need? How many hours of sleep is required to remain healthy? There is no right answer to these questions. The amount of sleep you need each night totally depends upon your body, over health status and schedule. Therefore, the answer to the above questions will vary. I usually get 6 to 7 hours of sleep each night. A lot of people feel that you need to receive 8 full hours of sleep every night. However, I am able to function properly without 8 full hours of sleep.
Infants and children usually sleep more than adults do. REM sleep decreases during the adolescence and young adulthood years and adults normally receive less sleep as they reach older age (www.sleepdisorderchannel.com, 2006).
Stages 3 and 4 of NREM, becomes extremely shorter in older people compared to other ages. Therefore, older people actually get fewer hours of deep sleep than younger people (www.sleepdisorderchannel.com, 2006). In addition, older individuals normally enter REM sleep quicker and are able to stay there longer (www.sleepdisorderchannel.com, 2006).
Cultural and biological factors have a great impact on infants, children, and adolescents sleeping patterns. Some sleep factors such as co-sleeping, bedtime rituals, the sleeping environment, napping, and parental expectations are influenced by cultural factors (Owens, 2004). According to Owens (2004), sleep problems are “grounded both in the physical, social, and biotic ‘micro-ecology’ of human sleep and the ‘macro-ecology’ the structure and organization of daily life in different cultures.” As a result, there is certainly no right way, particular place, or specific time to sleep (Owens, 2004).
It is common to see sleep problems in children. It is estimated that 25% of children experience a sleep problem during childhood (Owens, 2004). During the 1990’s American infants (3 months in age), slept 13 hours per day. Dutch infants, the same age, slept approximately 15 hours per day (Owens, 2004).
Another research study showed that Italian preschool children during the 1990’s went to bed later and woke up earlier, compared to children in other countries. Italian children reported attending evening social events with their parents. This resulted in them going to bed later than children of other cultures.
According to Owens (2004), a vast number of Icelandic children cease napping when they reach age three. However, American children usually take naps up until age 4 or 5 (Owens, 2004). In one study, there were differences in napping patterns between black and white 2-8 year olds. These children were from southern Mississippi. The black children napped more days each week, and slept less on weeknights than on weekends, when compared to the white children in the study. However, the total amount of sleep duration was almost the same between the two groups of children (Owens, 2004). Owens (2004), also states that children living in a “chaotic home environment,” are at-risk of suffering from sleep problems.
Many older people have problems sleeping at night. Elderly people find it more difficult to stay awake during the daytime, therefore they take more naps (Gammack, 2005). Many seniors in nursing homes, find it difficult to sleep at night and older individuals experience frequent awakenings during the night. It also takes them more than 30 minutes to fall asleep (Gammack, 2005). These sleep problems can result in illnesses, confusion and impaired functioning.
Research shows that exposure to an artificial light box, can significantly help improve sleeping patterns. This therapy usually requires 30 to 60 minutes of light exposure daily, for two or possibly three weeks (Gammack, 2005). There was a pilot study conducted at a 100 bed nursing home associated with a medical school. This study was designed to measure sleep outcomes in the elderly by using the natural light treatment. In this study, a method similar to artificial light research was administered to patients. The patients were exposed to 60 minutes of natural light therapy every morning for three weeks in a row. The control group stayed inside of a lit room with windows.
All patients were required to provide important information such as age, medication use and medical history. In addition, all participants were required to complete the “Sleep Scale Questionnaire.” A total of 11 people agreed to take part in this research study, with the average age was 81 years old. There were no differences found between the two groups, at the beginning or end of the survey. The study produced a positive correlation between the two groups. The intervention group found improvement in their sleep quality. As a result, this study suggests that “natural light improves sleep” (Gammock, 2005).
Depriving yourself of sleep can be very harmful to your health. Sleep deprivation is a serious issue with heavy consequences. Long periods of sleep deprivation has a negative impact on your metabolism and causes hunger (McClearly, 2006). Lack of sleep can also interfere with the way the body handles insulin. “Insulin resistance” puts a person at high risk of becoming diabetic or gaining weight (McClearly, 2006). It is extremely dangerous for anyone to go for extended periods of time without sleep. Research has proven that we need sleep in order to survive. I feel that a good night’s sleep is just as important as eating and drinking water. No one can continuously go days and weeks without sleep and live normal, effective lives.
In 2004, several researchers at the University of Chicago, conducted a study, which included four men. These men were restricted to only 4 hours of sleep each night. After two nights, the four subjects displayed an 18% loss in the hormone leptin. Leptin, informs your brain that you are full (McCleary, 2006). In addition, the men had a 28 percent increase in the hormone called ghrelin, which is the hormone that initiates hunger (McCleary, 2006).
The results of this study were reinforced by another study of approximately 10,000 adults. These adults discovered that individuals who slept less than 7 hours each night were very likely to be severely overweight compared to individuals who received the 7 hours of sleep a night (McCleary, 2006). The results of these studies suggest that sleep deprivation can cause weight gain, which can lead to serious health problems.
Ananthaswamy (2006), states that sleep deprivation can be very dangerous to the body, if humans respond to it like rats appear to do. A group at the University of Helsinki, Finland, observed sleep deprivation in rats. This group was led by Tarja Porkka-Heiskanen. Under Porkka-Heiskanen’s leadership, the team found out that the rats developed the molecules that are affiliated with stress. This information led the team to believe that sleep deprivation could possibly cause “stress-related illness,” like heart disease (Ananthaswamy, 2006).
Research suggests that chronic sleep deprivation can lead to weight gain, stress, and illness. This data proves how crucial adequate sleep is for the human body. I could not imagine constantly staying up late at night and having to go to work the next day. Lack of sleep would affect my responsibilities to my family, school and church. It would be very difficult for me to do certain tasks without getting a proper amount of sleep each night.
Lack of sleep could very well affect a person’s ability to drive a vehicle, operate machines, read a book, perform a dance routine, or complete any task in general. What if you went to the dentist to have a tooth pulled, and you found out that the doctor had not slept in two or three days. What would be your reaction? In addition, sleep deprivation may also affect your appearance. I have seen people develop wrinkles or “bags” under their eyes due to lack of sleep. So, it definitely pays to get a good night’s sleep.
According to McCleary (2006), there are a couple of ways to know if your body is not receiving enough sleep. When people fall asleep in just 1 or 2 minutes, they are probably suffering from sleep deprivation (McCleary, 2006). Another sign of sleep deprivation is sleeping for long periods of time during the day. “Chronic daytime sleepiness is not normal” (McCleary, 2006). Basically, we all need to sleep and there is no substitution for it.
Tips for Getting A Good Night’s Sleep
As I previously mentioned, it is important to get a good night’s sleep. There are some things that you can do to ensure that you receive adequate sleep. First, I will begin by saying that it is very important to eat healthy and exercise regularly. A good diet along with exercise will make you feel a lot better. And you will probably feel less stressed too. There was a time when I ate a lot of junk food, and as a result, I couldn’t always sleep well because I was tossing and turning from stomach aches or running to the bathroom because I was sick. I later realized how eating all of that junk food was preventing me from getting a good night’s sleep.
Next, try to relax before going to bed. It is good to free your mind from worries and stress before going to bed and it is best to go to bed with a positive mind. Try to fill your head with pleasant thoughts. You really do not want to go to bed full of tension. From personal experience, tension and stress really affect your sleeping pattern.
Next, avoid alcohol consumption for at least 3 to 5 hours before you go to sleep (Ravishankar, 2006). Excessive alcohol can be hazardous to the brain (Ravishankar, 2006). According to Ravishankar (2006), even a small amount of alcohol can have a negative impact on the quality along with the total duration of REM sleep. Research also suggests that alcohol “suppresses deep sleep, produces sleep fragmentation, and relaxes the upper airway muscles” (Ravishankar, 2006).
Taking a warm bath can also prepare you for a good night’s sleep. A warm bath will help you relax and hopefully relieve any tension that is in your body. In addition, a warm bath will help you take your mind off of things (like work), and focus on yourself for a moment.
It also helps to have a comfortable sleeping environment. I am a very neat and organized person and I keep my bedroom in order. I can’t sleep comfortably in a junky room. My bedroom is suited to match my personality. When I go to bed, I feel that I have escaped into my own world. It is quiet and peaceful in my room, unless the television is on. I definetely have a pleasant sleep environment. If you have problems falling asleep, you can try listening to soft music or even watching television if you prefer. However, the television can be very distractful, so I would not suggest keeping it on if you really do not need to.
Establishing a sleep schedule is very essential to getting a good night’s sleep. You definitely should allow yourself enough time to acquire the proper amount of sleep that your body needs to function. Please remember that everyone’s body is different, therefore, you have to be the judge of how much sleep you actually need. In addition, try eliminating your bedroom from excessive distractions. For example, cell phones, computers, video games, etc. can be very tempting and distractful. Exclude certain items from the bedroom that might interrupt your sleep.
Lastly, if you are still having problems getting to sleep, you can drink a warm glass of milk or tea before going to sleep. This has helped me in the past. Just do not eat anything heavy before going to bed. I think that you just have to find out what works for you. Some people have certain rituals that they do before going to sleep, such as meditating. Also, some people prefer sleeping in certain positions. You just have to experiment to discover what sleep pattern works for you.
This paper discussed sleep and the human brain. I addressed the sleep cycle and other important aspects of sleep. I looked at how the brain operates during sleep, the sleep stages and measurements, sleep deprivation, dreams and circadian rhythms. All of the sections included in this paper are significant to the study of sleep. Researchers and scientists, have spent many years studying the sleep cycle. I learned a lot of helpful information while researching this topic. I was amazed at some of the things that I discovered about sleep and the brain. I did not know that the brain was so active while we slept. I was intrigued with the material that I read on this topic.
Some aspects of sleep are still a mystery to us, especially dreams. We know for sure that sleep is necessary for survival and all humans need to sleep. The amount of sleep needed varies between individuals, but it is still mandatory that we receive an adequate amount of sleep each night.
This paper discussed the consequences of sleep deprivation. It is not wise to go for long periods of time without sleeping. Sleep deprivation can be very harmful to your mental and physical health. Lack of sleep can have a negative impact on your life. Without sleep, it would be hard to function and complete tasks each day. Without sleep, you will probably find yourself irritable, nervous and confused. So why put yourself through the frustration and pain?
I also addressed the benefits of sleep. There are so many benefits for getting a good night’s sleep. Sleep prevents certain diseases, fights bacteria, and it allows your brain to repair and rest. In addition, a good night’s sleep makes you feel refreshed and renewed. Also, sleep gives you the energy that you need to make it through the next day.
Thanks to research, we have a better understanding of why and how we sleep. Research studies have shown us so many things about the sleep process and how it works. The sleep cycle is truly amazing and I am motivated to continue studying sleep and dreams. I hope to make great contributions to these areas some day.
Ananthaswamy, Anil (2006). Why we’re not immune to losing sleep. New Scientist, 191(2569), 9.
Bird, Barbara (2005). Understanding dreams and dreamers: An adlerian perspective, 61(3), 200-216.
Gammack, Julie K. (2005). Sleep and sunlight in the elderly. Asia Pacific Biotech News, 9(15), 702-704.
Izac, Suzette Marie (2006). Basic anatomy and physiology of sleep. American Journal of Electroneurodiagnostic Technology, 46(1), 18-38.
McCleary, Kathy (2006). In search of sleep. Health, 20(7), 148-154.
Porte, Helen Sophrin (2004). Slow horizontal eye movement at human sleep onset. Journal of Sleep Research, 13(3), 239-249.
Siegel, Jerome M. (2005). Functional implications of sleep development. Plos Biology, 3(5), 756-758.
Siegel, Jerome M. (2003). Why we sleep. Neuroscience, 289(5), 92-97.
Someren, Eus J. W. Van (2000). Circadian rhythms and sleep in human aging. Chronobiology International, 17(3), 233-243.
Ravishankar, G. (2006). Is sleep a dormant state of mind? Positive Health, 123, 19-21.
Owens, Judith A. (2004). Sleep in children: Cross-cultural perspectives. Sleep and Biological Rhythms, 2, 165-173.
Sleep stages (2006). www.sleepdisorderchannel.com