The cycle between REM and NREM sleep is likely due to the fine interplay these two forms of sleep have on each other on memory formation. NREM sleep appears to do the work of removing unnecessary neural connections, while REM sleep does the work of strengthening neural connections. The disproportionate distribution of REM, will mean those waking up with limited sleep may only be missing out on a small proportion of total sleep but a large portion of REM sleep will be lost. Contrastingly, it is also possible to loose a higher proportion of NREM sleep over REM sleep, as we mention several times throughout the course of this article Those in there mid to late forties have shown a 60-70% reduction in deep NREM sleep, and those in there 70s have shown an 80-90% reduction in deep NREM sleep, which may be the reason for many neurological and general health issues associated with older age, especially in combination with an overall decreased total sleep time due to a weakened circadian rhythm and release of melatonin. The reason for the reduction in deep NREM sleep in the elderly is believed to be due to degeneration in important regions of the brain important for deep NREM generation, one possible reason for this is due to beta-amyloid proteins.

REM sleep is the stage of sleep where dreaming occurs, during this stage of sleep there are 4 main clusters of brain areas which spike in activity, these being the visuospatial regions which enable complex visual perception, the motor cortex which instigates movement, the hippocampus and its surrounding regions which support the autobiographical memory and the deep emotional regions of the brain (the amygdala and cingulate cortex), these deep emotional regions are activated even more so than what they are when awake, by up to 30%, monitoring the activation of these 4 regions of the brain during dreaming can be used to help very broadly predict the nature of the dream an individual is experiencing. Also during dreaming there was a significant decrease in activation of the far left and right side of the prefrontal cortex, these regions are responsible for rational thought and logical decision making, these regions send signals to other parts of the brain responsible for our emotions. Largely responsible for the contents of our dreams is the high activation of the hippocampus, which is likely the reason for the similar events and emotions we experience in our dreams compared to recent experiences when awake. It is still possible to have a form of dreaming in any form of sleep although, the kind experienced in NREM are not typical to what is classed as dreaming, but sometimes individuals may report thinking of certain things during NREM sleep once they have awoke.

The evolutionary purpose of REM sleep is unknown, it in fact may simply just be a byproduct of the neural activity which occurs during REM sleep. Fascinatingly REM sleep is the only time when the brain is completely shut off from noradrenaline, this has lead to the theory that dreaming in REM sleep may be useful in relieving highly emotional daytime experiences as a form of therapy. It is clear the evidence that sleep has towards healing the brain following a traumatic event, it is responsible for reducing the reactivity of the amygdala and increasing the activity of the rational prefrontal cortex in response to experiences which may trigger memories of the traumatic event. Further evidence suggests that REM sleep dreaming of traumatic events which have lead to depression (such as a bitter divorce) is necessary for the healing process of this mental disorder, the reason for this is likely because those who can not cure their depression are experiencing too high levels of noradrenaline to be able to enter and maintain REM sleep dreaming. The medication prazosin can help to lower noradrenaline levels in the brain and decouple traumatic memories from emotion whilst dreaming, this can be used to help treat mental disorders and decrease the risk of nightmares caused by past traumatic events. 

NREM sleep helps strengthen memories while REM sleep has the role of blending these memories, creating a unique way to solve problems and understand complex topics which are hard to be taught in an understandable way, such as the laws of grammar in the english language. When awoken from the sleep the neurochemical concentrations in the brain associated with sleep linger for some time, this is what is known as “sleep inertia”. Once awoken forcefully the brains neurophysiology is far more sleep like than awake like, with each passing the minute the brain enters a more awake state. When awoken from REM sleep individuals appear to immediately have a greater creative ability for problem solving, it appears that creative problem solving is greatest when REM sleep dreaming is associated with the problem at hand.

A common question asked is what is more important REM sleep or NREM sleep, evidence in rats suggest that around 40 days without either form of sleep often results in death, with deprivation of REM sleep usually causing death a few days sooner than deprivation of NREM sleep. This shows the importance of both forms of sleep for not just health and wellbeing but survival. The rats in this study experienced a rapid weight loss as a result of a constant drop in core body temperature and an attempt to cope with this by raising metabolic rate and an impaired immune system, leading to infections and scars on their skin, but the final cause of death in the rats were due to a wide array of damage and problems involving the major organs of their body’s. Other evidence in rats suggest that the main cause of death due to sleep deprivation is due to a toxic bacterial infection in the blood stream which dealt damage to the organs of the body, this infection appears to have originated from the rats very own gut.

Why we sleep

The hippocampus works as a short term memory store for information learnt, sleep then serves the purpose of shifting memories from the short term memory store into the cortex which serves as a long term memory store and restores the hippocampus back to a state ready for more learning. The ability for the brain to restore its learning capabilities (not move memories into the cortex) appears to occur in stage 2 NREM sleep, specifically during the sleep spindle phase. Those aged 60-80 years old appear to have a 40% deficit in sleep spindles which is likely largely responsible for elderly individuals lack of ability to learn when compared to young individuals, especially when combined with elderly individuals general lack of NREM sleep due to a lack of sleep quantity. Prior to a night of sleep newly formed memories are retrieved from the hippocampus, but after a thorough night of sleep they are retrieved from the cortex, this could be the reason for the phenomenon where a skill is learnt overnight or when you remember something following a good night of sleep despite not knowing it the day prior. Interestingly as little as a 20 minute nap has shown to have this memory consolidation effect, as long as it contains enough NREM sleep. For motor skills it is the amount of stage 2 NREM sleep spindles which occur just above the motor cortex the better the learning is off a specific motor skill, this stresses the need for sleep for athletes especially when also considering the added benefit sleep has on overall performance and adaptions to training at the physiological level, as well as the significantly reduced injury risk by roughly half when getting over 8 hours of sleep per night as compared to under 7.

As we age the amount of deep NREM we experience decreases, this is linked to a decline in memory. Sleep disturbances appears to proceed alzheimer's disease by several years, serving as a warning sign or even a contributor to the disease itself. In addition to this over 60% of patients suffering from alzheimer's are also suffering from a sleep disorder. It seems clear of the link between poor quality sleep and memory loss, and even alzheimer's, the reason for this is likely due to the lack of glymphatic system drainage in the brain during sleep. The pulsing rhythm of deep NREM sleep helps to significantly increase the activity of the glymphatic system, in this phase of sleep the glial cells of the glymphatic system shrink by up to 60% increasing the space around the neurons and allowing cerebrospinal fluid to clear out metabolic waste products such as amyloid-beta proteins which accumulate throughout the day and are linked to alzheimer's disease. Amyloid-beta proteins have the ability to kill the surrounding brain cells. In alzheimer's disease amyloid-beta proteins appears to accumulate more in the middle part of the frontal lobe, this is the region essential for the electrical generation of deep NREM sleep. What remains a question is that if alzheimer's disease is typified by memory loss why does the hippocampus appear to be unaffected by amyloid-beta proteins, one theory is due to amyloid-beta proteins ability to reduce the amount of deep NREM sleep, this will cause an even greater accumulation in amyloid-beta proteins and thus cause even greater sleep disturbances.

Those who restrict sleep have been shown to have a 45% increased risk of heart disease. One reason for this is believed to be due to the rise in blood pressure and heart rate caused by just slight sleep reductions, this damages blood vessels especially the coronary arteries of the heart, this constant damage can cause a build up of plaque on the artery walls in a dangerous condition known as atherosclerosis. Individuals obtaining 5 to 6 hours of sleep per night have been shown to be 200-300% more likely to suffer calcification of the coronary artery over just a 5 year period. The increased blood pressure and heart rate associated with sleep deprivation is likely the cause of an overactive sympathetic nervous system and a rise in cortisol, which has the additional impact of narrowing the blood vessels, this is because deep NREM sleep is responsible for calming the sympathetic nervous system. On top of this, with a lack of sleep growth hormone does not have the chance to do its job of healing damaged blood vessels. An additional serious negative effect of sleep deprivation is the disruption to the guts microbiome due to high sympathetic nervous system release of cortisol which triggers the growth of bad bacteria in the gut, and poor enteric nervous system activity, which can prevent the efficient absorption of certain nutrients and cause gastrointestinal distress. The sympathetic nervous system is also responsible for increasing metabolic rate and therefore core body temperature, this makes it difficult to sleep as a drop in core body temperature is essential. To sleep the amygdala, hippocampus, thalamus and other alertness and vigillance regions of the brain ramp down in activation due to a decrease in sympathetic nervous system activity, this is exactly what commonly doesn’t happen in individuals suffering from insomnia. Even when an individual can fall asleep it is still not a sign of good high quality sleep, individuals can experience less powerful brain waves during NREM sleep, and irregular fragmented REM sleep often with frequent awakenings even if these go undetected.

Limiting sleep to just 4 hours per night for just 6 nights has been shown to reduce an individual's ability to absorb glucose by 40% by causing insulin resistance, add to this the increase appetite associated with sleep deprivation and it is clear that it is a recipe for prediabetes and diabetes causing an excessively high amount of glucose circulating in the blood stream which overtime can deal serious harm to the tissues and organs of your body. Such symptoms include eye disease, nerve disease, kidney failure, hypertension and heart disease, as well as overall damage across the body causing an averaged 10 years decrease in life span. Not only is sleep deprivation strongly linked to obesity but also there is a 20% greater loss in muscle mass in those who go on a very strict low calorie diet with limited sleep over those getting a healthy amount of sleep, this is not to mention the difficulties of falling asleep while on a typical low calorie diet, although sleeping on a completely full stomach has also been proven to be difficult.

Sleep deprivation has also been shown to have a devastating impact on reproductive health, men show decreased testosterone levels, a 29% lower sperm count and a higher rate of deformities in sperm. There is a long list of symptoms associated with low testosterone (hypogonadism) these include lethargy, lack of concentration, reduced sex drive, decreased bone density, reduced muscle mass and rate of muscle growth. Woman are also vulnerable to the reproductive issues caused by sleep deprivation. Woman sleeping less than 6 hours per night show a 20% reduction in follicular-releasing hormone, this hormone is necessary for conception, this may be partly the reason for the 80% greater rates of subfertility and menstrual cycle irregularities associated with sleep deprivation.

Sleep also appears to be incredibly important towards the immune system. Those sleeping 5 hours per night were 50% likely to catch the common cold when exposed to it, as opposed to 18% for those sleeping 7 hours per night. Sleep prior to vaccines is also incredibly important, those who sleep roughly 8 hours per night in the week prior to getting the flu shot generated a powerful antibody response, while those sleep deprived had only 50% of the immune reaction to the flu shot. The immune related benefits of sleep goes beyond the flu however, natural killer cells are responsible for helping remove cancer cells by injecting a certain protein which can destroy them, sleep deprivation has been shown to decrease the quantity of these very type of cells. A single night of 4 hours of sleep has proven enough to decrease circulating killer cells by 70%. The immune issues associated with a lack of sleep can be attributed to an overactive sympathetic nervous system stimulating an inflammatory response. Inflammation can also increase the risk of cancer itself and be used to help aid the growth of blood vessels which nourish the tumour. Inflammation may also be partly responsible for moving parts of the tumour and allowing it to spread. Evidence in mice suggest that sleep deprivation can increase the rate of cancer growth to 300% and make the cancer significantly more likely to spread, in the same mice it was found that the macrophage M1 significantly decreased, which is responsible to help combat cancer cells. The macrophage M2 also significantly increased, which is responsible for promoting the growth of cancer cells.

Sleep deprivation has also been shown to have a significant impact of gene expression and suppression. When sleep deprived genes involved in chronic inflammation, cellular stress and various factors involved in cardiovascular disease all increase, while genes involved in stable metabolism and the immune response are suppressed. On the note of cardiovascular disease, sleep deprivation has also been linked to a drop in HDL which increases the risk of cardiovascular disease. One reason for the altered genetic state caused by sleep deprivation is due to damage of the telomere which functions as a protective cap on the end of DNA strands, this can leave the DNA exposed and vulnerable to undesirable alterations.

Methods of enhancing sleep and practical implications

Placing electrode pads on an individuals head and timing an electrical voltage with the slow waves of NREM sleep has been shown to have doubled the individuals ability to learn new facts, similar studies performing the same thing during REM sleep or during wakefulness have not been shown to cause a beneficial memory consolidating effect. Other methods includes monitoring brain waves and playing quiet tones next to the sleeper in time with the waves of NREM sleep, this has shown to have a 40% memory enhancement effect, however, timing these quiet tones just off the natural peak of slow brain waves have been shown to disrupt sleep quality, similarly, gentle rocking of a bed has been shown to boost the quality of slow brain waves and double the number of sleep spindles.

Fascinatingly it has been shown that playing a corresponding sound to specific information and then replaying the same sound at a quiet volume during sleep significantly helps to form the memories of what has been learnt, this method may be the single best sleep memory consolidation technique which can be applied in a practical setting, for instance, playing music with a clear beat during learning and repeating this same sound over a full night of sleep may significantly help to aid in learning without any downsides.

A fascinating discovery found that during sleep the brain has the ability to better retain information which has been told to be remembered than information which is said to be unimportant, even when without sleep the memory consolidation of both remains the same. This is because during NREM sleep the frontal lobe works with the hippocampus to move memories from the short term storage site to the long term storage site in the cortex. The frontal lobe here works as a filter for unimportant information. The ability for the brain to select information to remember and forget may have practical implications in situations such as trauma and drug addiction.

Evidence suggests that a single night of sleep deprivation following learning and you have significantly missed out on an opportunity to consolidate these memories even if in the following nights you was to achieve 8 or more hours of sleep per night. This stresses the importance of sleep following learning. Even though cramming for an exam and limiting sleep may yield good short term results the information will be retrieved from the hippocampus rather than the cortex which serves as the long term memory store, so the best learning method is obvious, do not cram information, make an effort to study daily long before an important exam and achieve a minimum of 8 hours of sleep each night.

Daylight contains all wavelengths of light, from the short wavelengths which appear as blue and purple to the longer wavelengths which appear as orange and reds, the loss of daylight means the suprachiasmatic nucleus will stop the brake pedal on the pineal gland and melatonin can be unleashed. However artificial lights enter the receptors of the eyes and stops the appropriate release of melatonin, this is the case for even dim lights such as those coming from a bedside lamp. The light receptors of the eye appear to be more sensitive to more short wavelength lights, this is the exact kind of light emitting in large amounts from LED light, the kind found in most smartphones and TVs. The use of an ipad for several hours before sleep appears to suppress melatonin release by about 50% and delays the release of melatonin by up to 3 hours, and therefore made it more difficult to fall asleep, acquire sufficient REM sleep once falling asleep and it delays the release of melatonin for several days after the day of ipad use. Due to its prevalence there is little we can do to combat LED lights harmful impact on sleep. The best recommendations is to install software on any devices which are used at night which block blue light, and try to limit exposure to LED light at night and even during the night.

For those serious about sleep, alcohol should also be avoided. Alcohol functions by binding to receptors in the brain and stopping them from firing their electrical impulses, this occurring in the prefrontal cortex is responsible to the increased sociability associated with alcohol consumption, while this occuring in the prefrontal cortex is responsible for the tiredness associated with alcohol consumption, although sleep caused by alcohol consumption, shows very different brainwave activity to normal sleep and it certainly does not have as much of its beneficial effects. Alcohol has been shown to cause more awakenings through the night and suppress REM sleep, alcohol does this because as it gets metabolised in the night it forms the byproducts aldehydes and ketones, of this aldehydes in particular have been shown to block the ability for the brain to form REM sleep. Alcohol consumption the night of learning new information has been shown to reduce the ability to recall this information by about 50%, fascinatingly consuming alcohol 3 nights after learning information, despite 2 nights of normal sleep still reduced the ability to recall newly learnt information by 40%. Unfortunately there is little we can do to prevent these negative effects alcohol has on memory formation, the best advice that can be offered is to avoid alcohol as much as possible, at least around periods where much information needs to be remembered such as in the weeks prior to an exam.

Evolutionary the relationship with light and melatonin will help an individual become ready for sleep following sunset, core body temperature has a similar relationship with sleep, because as the sun sets air temperature typically gets cooler and so does core body temperature. A drop in core body temperature is detected by thermosensitive cells located in the center of your brain within the hypothalamus this signals to the next door suprachiasmatic nucleus which signals to the pineal gland to release melatonin. The head, hands and feet are rich in blood vessels and arteriovenous anastomosis which radiate heat and cool the body. Selectively warming the hands and feet have been shown to bring warm blood out of the core of the body and cool it allowing individuals to fall asleep 20% faster, even if the individual already has no sleep issues. The use of specialised whole body thermal sleeping suits have also allowed us to find that by keeping the core body temperature cool throughout the night there is a very significant decreased chance of waking up through the night and even the deep powerful brain waves of NREM sleep are boosted by this effect. Most of us do not have access to special sleeping suits but a hot bath prior to sleep has been shown to induce sleep faster and induce more deep NREM sleep by 10-15%, this is because the hot bath stimulates blood flow to the skin and dilation of the blood vessels this means when exiting the bath core body temperature will drop.

Waking up to an alarm clock in the morning activates the sympathetic nervous system causing a rise in heart rate and blood pressure, many individuals have this experience multiple times every morning, add this up over many years and it seems inevitable that this isn’t healthy. The best way to not have the need to rely on an alarm clock in the morning is to go to bed at the same time and set your alarm for the same time every day, making sure to get up at the first alarm, in time the alarm may not even be needed if you allocate yourself enough of a sleep opportunity, which is typically 7-9 hours. 18.3°C appears to be the best bedroom temperature on average for sleeping.

Melatonin appears to have no more than a placebo effect for young individuals although it may be useful in “resetting” the circadian rhythm for individuals if they change time zone. Sleeping pills, like alcohol are classed as sedative drugs, that means rather than causing normal healthy sleep they cause more of a sedation like effect stopping electrical impulses from firing. Sleeping pills does this to the higher regions of the cortex of the brain, and once falling asleep the deep NREM brain waves are lacking. Those who take sleeping pills may believe they are obtaining normal healthy sleep and they therefore do not try and fix any habits which may be making it difficult for them to achieve normal healthy sleep which has shown to be more effective, evidence even suggests that, like melatonin tablets,sleeping pills are little more effective than placebo. Another caveat to sleeping pills is the resistance caused by them making higher doses more necessary and the quality of sleep decreases, when the individual goes of the sleeping pills it can also leave a dependency on sleeping pills making it even harder to fall asleep naturally. There is a linear relationship between sleeping pill consumption overtime and risk of mortality, one of the more prevalent problems that appears to be due to more than the sleeping pills effect on sleep quality but sleeping pills ability to blunt the immune system.

Amongst the many suggestions to improve sleep quality it is important to mention the recommendation to not sit in bed awake for a prolonged period of time, it is recommended to get into bed once tired and if you can not sleep remove yourself from the bed, this is in order to keep the psychological association with your bed and sleep rather than sitting on your phone, watching TV or doing work. Approaching sleep it is also important to relax and try not to take any naps in the evening which may prevent tiredness when night time comes.

A common sleep issue is sleep apnea. Obstructive sleep apnea is caused by a blockage of the upper airway often due to obesity or enlarged adenoids and tonsils, while central sleep apnea is caused by an ability to signal the body to breath during sleep. Sleep apnea can often be detected by loud, rough snoring, often littered with periods of breathlessness. The inability to breath while asleep due to sleep apnea can begin to starve the body of oxygen, to compensate for this the individual will often awake, sometimes noticeably, in order to breath correctly. Sleep apnea prevents the ability of sustaining or possibly even reaching a state of deep NREM sleep, which as you can imagine over years can be incredibly harmful, this is because in deep NREM sleep the upper airway appears to relax and narrow more than in other stages. Losing weight, sleeping on your side, physical activity and not smoking are all ways which may help prevent sleep apnea, although those who believe they are suffering from this condition should seek medical advice incase it is a sign of an underlining medical condition, you may need surgery to deal with the condition or you may need medication or specialised equipment in order to regain normal health sleep.