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Why do we wake up after 6-8 hours of sleep ? Why don't we sleep for lets say 20-30 hours ? What mechanism is it that controls when to wake us up and how does it determine the moment that we should wake up ?
Think this review from Nature can answer your question. A brief summary ( with slight simplification):
The ascending activating system (that is responsible for wakefulness) has two branches:
The first one ascends to the thalamus and activates the thalamic relay neurons - these transmit information to the cortex. They get their main input form two acetylcholine producing group of cells: the pedunculo- pontine and laterodorsal tegmental nuclei (PPT/LDT). These two group of cells are also the source of main input for reticular nucleus of the thalamus.
The neurons in the PPT/LDT fire most rapidly during wakefulness and rapid eye movement (REM) sleep, which is the stage accompanied by cortical activation, loss of muscle tone in the body and active dreams
The second branch, directly activates neurons in the hypothalamus, basal forebrain (BF) and cortex, bypassing the thalamus. This pathway has origins in monoaminergic neurons in the upper brainstem, caudal hypothalamus, as well as the noradrenergic locus coeruleus (LC), serotoninergic dorsal (DR) and median raphe nuclei, dopaminergic ventral periaqueductal grey matter and histaminergic tuberomammillary neurons. Input from this pathway to the cortex is extended /augmented by the lateral hypothalamic peptidergic neurons (that contain melanin-concentrating hormone (MCH) or orexin/hypocretin), and BF neurons(that produce acetylcholine or GABA).
Sleep is promoted by the so called VLPO (ventrolateral preoptic nucleus). The VLPO send outputs to major group of cells of the arousal system in the hypothalamus and brainstem.
The VLPO neurons are primarily active during sleep, and contain the inhibitory neurotransmitters, galanin and GABA. The VLPO also receives afferents from each of the major monoaminergic systems. Both noradrenaline and serotonin inhibit VLPO neurons. The latter do not have histamine receptors, but the tuberomammillary neurons also contain GABA, which is inhibitory to VLPO neurons, as well as several other potentially inhibitory pep tides, such as galanin and endomorphin. Therefore, the VLPO can be inhibited by the very arousal systems that it inhibits during sleep.
As you can see in the quote from the paper these two systems can mutually inhibit each other and basically work as a seesaw or 'flip-flop switch'. In this system activity in one subsystem shuts down the inhibition from the other subsystem and disinhibits its own action (selfreinforcing loop). The transition from one state to the other is relatively fast. Orexin plays important role in stabilizing the switch in one position or the other.
Regulation pf sleep:
Homeostatic: It is unclear how homeaostatic regulation of sleep work, yet it has been proposed, that adenosine accumulation in the brain may signal for 'need for sleep'. The extended wakefulness causes the exhaustion of brain glycogen reserves and depletion of ATP levels. As ATP is degraded to ADP, AMP and eventually adenosine, extracellular adenosine levels rise in some parts of the brain including the BF.
In addition, adenosine might disinhibit the VLPO via presynaptic A1 receptors by reducing inhibitory GABAergic inputs.
This implies that during sleep ATP and energy sources are regenerated and free adenosine levels drop, thus inhibition of VLPO might take place again. Also it is important to note that the amount of sleep we need is proportional to the time we spend awake.
It is known that humans have quite precise :circadian rhythm:
The suprachiasmatic nucleus (SCN) serves as the brain's 'master clock'. Neurons in the SCN fire in a 24-hour cycle that is driven by a transcriptional-translational loop, which persists even when the neurons are dissociated in cell culture.
The SCN has moderate output to the VLPO or orexin neurons, but it has more important projections to the adjacent subparaventricular zone (SPZ) and the dorsomedial nucleus of the hypothalamus (DMH).
The SPZ also has relatively limited projections to the VLPO, orexin neurons and other components of the wake-sleep-regulatory system. However, a major target is the DMH. This region receives inputs from many more neurons in the SPZ than the SCN, so the SPZ is in a position to amplify the output of the SCN. (… ) The DMH is one of the largest sources of input to the VLPO and orexin neurons and is crucial for conveying SCN influence to the wake-sleep-regulatory system. The DMH projection to the VLPO comes largely from GABA-containing neurons (that is, those that promote wakefulness by inhibiting sleep), and the projection to the LHA originates from neurons containing glutamate and thyrotropin-releasing hormone (which should presumably be excitatory and promote wakefulness).
So as you can see our circadian rhythm as direct control over our sleep-wake cycle, and that the SCN can heavily promote awakening through SPZ and DMH.
Thorough review and details can be found in the article I linked at the beginning, I only tried to select key information to provide a (not so ) brief summary. We only sleep about 7-8 hours normally because this is enough for homeostatic regeneration and our circadian rhythm promotes wakefulness in the morning (this is quite simplified).
Based on their abstract these limited access papers might also be useful (unfortunately I do not have full access to them right now):
I think what the OP is getting at is less about the mechanism of sleep -> wake transition and more about how the length of sleep time is regulated…
In that regard, it's an interesting question without a definitive answer. In a sense, it is a question of homeostasis and the time when your sleep need/pressure is satisfied determines when you wake up (without an alarm clock).
It is hypothesised that sleep drive is a function of the previous wake period plus any sleep debt you carried from the previous days. One molecule, adenosine, is thought to represent that and the accumulation of that in the brain makes you want to sleep [Ref. 1]. Thus, the clearance of external adenosine in the brain to a certain level may be part of the homeostatic mechanism signalling enough sleep.
Another hypothesis is that sleep is required to encode memories long term [Ref. 2] and to calibrate emotion/mood [Ref. 3] which themselves can add to the sleep need and those processes would need to be completed to a certain level to release that sleep need.
As for the specific moment for awakening, if there isn't some external arousal stimulus like daylight/alarm or the rush of cortisol cycle driven by your circadian mechanism, then it will likely correlate to the end of the current 90min sleep cycle when your brain is close to the wake state coming out of REM [Ref. 4]. In fact, it's thought you wake a little every 90min through the night, it's just you don't remember it.
[Ref. 1]: Link to adenosine and sleep review [Ref .2]: Link to sleep and memory review [Ref. 3]: Link to sleep and mood review [Ref. 4]: Link to brief summary of sleep staging
The basic physiology and pathophysiology of melatonin
Melatonin is a methoxyindole synthesized and secreted principally by the pineal gland at night under normal environmental conditions. The endogenous rhythm of secretion is generated by the suprachiasmatic nuclei and entrained to the light/dark cycle. Light is able to either suppress or synchronize melatonin production according to the light schedule. The nycthohemeral rhythm of this hormone can be determined by repeated measurement of plasma or saliva melatonin or urine sulfatoxymelatonin, the main hepatic metabolite. The primary physiological function of melatonin, whose secretion adjusts to night length, is to convey information concerning the daily cycle of light and darkness to body physiology. This information is used for the organisation of functions, which respond to changes in the photoperiod such as the seasonal rhythms. Seasonal rhythmicity of physiological functions in humans related to possible alteration of the melatonin message remains, however, of limited evidence in temperate areas in field conditions. Also, the daily melatonin secretion, which is a very robust biochemical signal of night, can be used for the organisation of circadian rhythms. Although functions of this hormone in humans are mainly based on correlative observations, there is some evidence that melatonin stabilises and strengthens coupling of circadian rhythms, especially of core temperature and sleep-wake rhythms. The circadian organisation of other physiological functions could depend on the melatonin signal, for instance immune, antioxidative defences, hemostasis and glucose regulation. Since the regulating system of melatonin secretion is complex, following central and autonomic pathways, there are many pathophysiological situations where the melatonin secretion can be disturbed. The resulting alteration could increase predisposition to disease, add to the severity of symptoms or modify the course and outcome of the disorder.
The relationship between sleep and cognitive function has been a topic of interest for over a century. Well-controlled sleep studies conducted with healthy adults have shown that better sleep is associated with a myriad of superior cognitive functions, 1,2,3,4,5,6 including better learning and memory. 7,8 These effects have been found to extend beyond the laboratory setting such that self-reported sleep measures from students in the comfort of their own homes have also been found to be associated with academic performance. 9,10,11,12,13
Sleep is thought to play a crucial and specific role in memory consolidation. Although the exact mechanisms behind the relationship between sleep, memory, and neuro-plasticity are yet unknown, the general understanding is that specific synaptic connections that were active during awake-periods are strengthened during sleep, allowing for the consolidation of memory, and synaptic connections that were inactive are weakened. 5,14,15 Thus, sleep provides an essential function for memory consolidation (allowing us to remember what has been studied), which in turn is critical for successful academic performance.
Beyond the effects of sleep on memory consolidation, lack of sleep has been linked to poor attention and cognition. Well-controlled sleep deprivation studies have shown that lack of sleep not only increases fatigue and sleepiness but also worsens cognitive performance. 2,3,16,17 In fact, the cognitive performance of an individual who has been awake for 17 h is equivalent to that exhibited by one who has a blood alcohol concentration of 0.05%. 1 Outside of a laboratory setting, studies examining sleep in the comfort of peoples’ own homes via self-report surveys have found that persistently poor sleepers experience significantly more daytime difficulties in regards to fatigue, sleepiness, and poor cognition compared with persistently good sleepers. 18
Generally, sleep is associated with academic performance in school. Sleep deficit has been associated with lack of concentration and attention during class. 19 While a few studies report null effects, 20,21 most studies looking at the effects of sleep quality and duration on academic performance have linked longer and better-quality sleep with better academic performance such as school grades and study effort. 4,6,9,10,11,12,13,22,23,24,25,26,27 Similarly, sleep inconsistency plays a part in academic performance. Sleep inconsistency (sometimes called “social jet lag”) is defined by inconsistency in sleep schedule and/or duration from day to day. It is typically seen in the form of sleep debt during weekdays followed by oversleep on weekends. Sleep inconsistency tends to be greatest in adolescents and young adults who stay up late but are constrained by strict morning schedules. Adolescents who experience greater sleep inconsistency perform worse in school. 28,29,30,31
Although numerous studies have investigated the relationship between sleep and students’ academic performance, these studies utilized subjective measures of sleep duration and/or quality, typically in the form of self-report surveys very few to date have used objective measures to quantify sleep duration and quality in students. One exception is a pair of linked studies that examined short-term benefits of sleep on academic performance in college. Students were incentivized with offers of extra credit if they averaged eight or more hours of sleep during final exams week in a psychology class 32 or five days leading up to the completion of a graphics studio final assignment. 33 Students who averaged eight or more hours of sleep, as measured by a wearable activity tracker, performed significantly better on their final psychology exams than students who chose not to participate or who slept less than eight hours. In contrast, for the graphics studio final assignments no difference was found in performance between students who averaged eight or more hours of sleep and those who did not get as much sleep, although sleep consistency in that case was found to be a factor.
Our aim in this study was to explore how sleep affects university students’ academic performance by objectively and ecologically tracking their sleep throughout an entire semester using Fitbit—a wearable activity tracker. Fitbit uses a combination of the wearer’s movement and heart-rate patterns to estimate the duration and quality of sleep. For instance, to determine sleep duration, the device measures the time in which the wearer has not moved, in combination with signature sleep movements such as rolling over. To determine sleep quality, the Fitbit device measures the wearer’s heart-rate variability which fluctuates during transitions between different stages of sleep. Although the specific algorithms that calculate these values are proprietary to Fitbit, they have been found to accurately estimate sleep duration and quality in normal adult sleepers without the use of research-grade sleep staging equipment. 34 By collecting quantitative sleep data over the course of the semester on nearly 100 students, we aimed to relate objective measures of sleep duration, quality, and consistency to academic performance from test to test and overall in the context of a real, large university college course.
A secondary aim was to understand gender differences in sleep and academic performance. Women outperform men in collegiate academic performance in most subjects 35,36,37,38 and even in online college courses. 39 Most of the research conducted to understand this female advantage in school grades has examined gender differences in self-discipline, 40,41,42 and none to date have considered gender differences in sleep as a mediating factor on school grades. There are inconsistencies in the literature on gender differences in sleep in young adults. While some studies report that females get more quantity 43 but worse quality sleep compared with males, 43,44 other studies report that females get better quality sleep. 45,46 In the current study, we aim to see whether we would observe a female advantage in grades and clarify how sleep contributes to gender differences.
What can I do to get better sleep?
- Stick to a regular sleep schedule. Go to bed at the same time each night and get up at the same time each morning, including on the weekends.
- Get enough natural light, especially earlier in the day. Try going for a morning or lunchtime walk.
- Get enough physical activity during the day. Try not to exercise within a few hours of bedtime.
- Avoid artificial light, especially within a few hours of bedtime. Use a blue light filter on your computer or smartphone.
- Don&rsquot eat or drink within a few hours of bedtime avoid alcohol and foods high in fat or sugar in particular.
- Keep your bedroom cool, dark, and quiet.
Work with your health care team to identify obstacles to good sleep, including other medical conditions.
How long is the ideal nap?
Taking a nap in the afternoon can serve as a reset button for some people, allowing them to wake up feeling refreshed and ready to finish their day. However, it may be best to aim for short 20-minute naps for the greatest benefit.
The actual time a person needs to sleep for during a nap may vary slightly depending on their age and personal sleep cycle.
That said, most people will feel best after a nap that does not dip too far into their sleep cycle.
In this article, learn more about how long a should be, as well as what the benefits are.
Share on Pinterest For the most benefit, a person should aim to nap for 20 minutes.
The National Sleep Foundation recommend taking a 20-minute nap to wake up feeling refreshed. The ideal nap duration can vary from person to person, but most professionals agree that shorter naps are better if a person’s goal is to wake up feeling refreshed and alert.
However, there may also be some benefit to longer naps. For instance, the results of a 2019 study indicated that 25-, 35-, and even 45-minute naps significantly reduced signs of stress and fatigue in physically active men. It also improved their attention and physical performance.
With this said, short naps, or “power naps,” can help a person feel more awake and refreshed.
The National Sleep Foundation warn that taking longer naps may leave a person feeling groggy, as they will need to wake up from a deeper sleep.
It is important to time naps well due to the sleep cycle. As a person sleeps, their brain naturally moves through different stages of sleep. These stages cause different brain wavelengths and release specific hormones into the bloodstream.
These effects can cause noticeable changes in a person’s waking state after taking a nap, depending on which stage they wake up in.
In the most beneficial naps, a person will only go into the first and second stages of sleep. These stages are more superficial and can help a person feel refreshed without them needing to go into a deeper sleep.
During a full night’s sleep, a person will go through their entire sleep cycle multiple times. For most people, the whole sleep cycle is somewhere around 90–110 minutes long .
Allowing the brain and body to reach the deep stages of sleep makes a person much less responsive to outside stimuli. It also causes the brain to release compounds that can make a person more tired, which helps them stay asleep for the whole night.
A person who wakes up from a nap feeling heavy and groggy likely went further into their sleep cycle. If this happens regularly after naps, they may want to check how long they are sleeping for and set an alarm.
Although about 15–20 minutes is ideal for adults, the best napping durations can vary by age.
For instance, newborns sleep most of the day, as their development takes a lot of energy. Young babies are also likely to take a few long naps throughout the day, which is good for their health.
Toddlers and young children will start to develop a regular sleep pattern, but they could still benefit from taking a nap in the afternoon. For example, they may respond well to longer naps of around an hour in the middle of their day.
Teenagers face many challenges that can make them feel tired, such as hormonal changes, studying, and early school start times. One 2019 study found that the best nap duration for teenagers is around 30–60 minutes.
Naps can be a healthful addition to a person’s day, as long as they are short and do not interfere with a person’s nighttime sleep.
The authors of a 2019 study note the long history of research that has found strong evidence of the health effects of napping, including:
- improving cognitive performance
- enhancing short term memory
- improving mood
- reducing sleepiness and fatigue
- boosting athletic performance
When it comes to long term health effects, there is nothing to suggest that napping is unhealthful in most individuals.
That said, some people may not benefit from naps. For instance, people who have sleep disorders such as insomnia may find that napping makes it more difficult for them to fall asleep at night.
Science Says This Is Exactly How To Nap To Be At Your Best
Is it good to take naps during the day? originally appeared on Quora: the knowledge sharing network where compelling questions are answered by people with unique insights.
It's great to take naps during the day, but only if you do it correctly. It is much better to take naps than to stay awake for extended periods of time attempting to finish all your tasks, because, let's face it, 8-9 hours of sleep a day is not always possible. Naps "reboot" your brain and help you approach your work with a fresh and clear state of mind.
You can choose the length of your nap in order to cater to your specific need. Yes, how long you should nap is based on your reason for napping.
The power nap is 10 to 20 minutes long. Take a power nap to quickly boost your energy and alertness. A power nap will help you get back to work right away. This is because this amount of sleep does not yet reach the deeper states of a sleep cycle and it should be easy to get up and work again. The napper stays in the lighter stages of NREM (non-rapid eye movement) sleep.
Avoid 30-minute naps. There are no significant benefits to this length of nap. Half-hour naps cause "sleep inertia," a groggy state than can last for about 30 minutes after waking up. This is because the body is forced awake right after beginning, but not completing, the deeper stages of sleep. Sleeping for 60 minutes includes the deepest type of sleep, slow-wave sleep. Because of this, the one-hour nap is ideal for helping an individual better remember faces, names, and facts. However, a sleep cycle will not be completed in only 60 minutes, so you may not be very alert for some time after waking up.
The most ideal nap is the 90-minute nap. Why? 90 minutes is the length of one full sleep cycle, which includes all the light and deep (REM and dreaming) stages of sleep. A full sleep cycle nap improves procedural and emotional memory (e.g. for playing musical instruments and driving). A 90-minute nap can also significantly boost one's creativity. Because the nap is a full sleep cycle, waking up should come much easier.
There are optimal times for napping throughout the day that depend on the time you wake up in the morning. If at all possible, try to plan your naps during these times.
According to sleep researcher Dr. Sara Mednick, the optimal napping time is the time of day where slow-wave sleep (deep sleep) and REM intersect. Use her interactive nap wheel to determine what time you should nap based on when you woke up. Below is an example nap wheel for a person that woke up 7 am. This person should nap at 2 pm (or 2 am if not yet asleep).
This question originally appeared on Quora. Ask a question, get a great answer. Learn from experts and access insider knowledge. You can follow Quora on Twitter, Facebook, and Google+. More questions:
Although researchers have learned a lot about sleep and sleep disorders in recent years, important questions remain, such as how sleep and circadian disturbances affect human health and how to best prevent, diagnose, and treat these disorders. In 2016, the NHLBI released its Strategic Vision, which will guide the Institute’s research activities for the coming decade. Many of the objectives, compelling questions, and critical challenges identified in the plan focus on sleep. For example, researchers will be looking at whether changing the time of day (circadian rhythm) when one sleeps, eats, and takes medicines can help improve existing treatments for other diseases, such as high blood pressure, diabetes, and asthma. Training the next generation of sleep scientists is also a high priority for NHLBI.
NHLBI will continue to work with its partners to translate scientific sleep research discoveries into improved strategies to prevent and treat sleep disorders. NHLBI is committed to working with researchers, health care providers, and public and private organizations to implement the research opportunities outlined in the NIH Sleep Research Plan. Recommended research initiatives include looking at the connection between sleep and the body’s natural circadian rhythm, studying the influence of genetic and environmental factors that could influence a person's sleep health, and conducting more clinical trials to improve treatments for sleep and circadian disorders.
Sleep health continues to be a nationwide health improvement priority in Healthy People 2030. Healthy People provides science-based, national objectives for improving the health of all Americans over a 10-year period. The sleep health goal calls for an increase in public knowledge about how adequate sleep and treatment of sleep disorders can improve health, productivity, wellness, quality of life, and safety on the roads and in the workplace. This will be accomplished by focusing on four objectives:
How Long It Takes to Break a Habit?
There&rsquos no magic number of repetitions that&rsquoll get you to internalize the habits you want. Researchers have proposed several different ways of understanding habit formation.
The 21-Day Rule (or Myth?)
One of the earliest and most popular pieces of literature on the subject is Psycho-Cybernetics (1960) by Maxwell Maltz. Dr. Maltz who was a plastic surgeon wanted to understand how people viewed themselves. In particular, he was curious about how long it took for patients to get used to changes he made during surgery.
Based on observing his patients and reflecting on his own habits, he determined that it took at least 21 days for people to adjust. He used this information as the basis for many &ldquoprescriptions&rdquo in his self-help oriented Psycho-Cybernetics. 
Since then, self-help gurus have latched onto the idea of taking 21-days to change habits. People began to forget that he said &lsquoa minimum of about 21 days&rsquo instead of &lsquoit takes 21 days to form a new habit.&rsquo
Give Yourself a Month?
Another popular belief in self-help culture states that habits take 28 to 30 days to form.
One proponent of this rule, Jon Rhodes, suggests: 
&ldquoYou must live consciously for 4 weeks, deliberately focusing on the changes that you wish to make. After the 4 weeks are up, only a little effort should be needed to sustain it.&rdquo
This was a generally agreed-upon figure, but the 21-day rule popularized by readers of Maltz was more appealing to many people because it was easy to understand, and it was faster than the general 28-30 rule.
If you want to know more about the myths of how long it takes to break a habit, check out this video:
The Time-Frame for Changing Habits Varies
While the 21 and 28-day rules appeal to our desire to change quickly, a 2009 study from University College London suggests that the window for change can be much wider. The research, published in The European Journal of Social Psychology, followed habit-formation in 96 people over a 12-week period.
The UCL study looked at automaticity, which is how quickly people engaged in the actions they wanted to turn into habits. Researchers explained: 
As behaviours are repeated in consistent settings they then begin to proceed more efficiently and with less thought as control of the behaviour transfers to cues in the environment that activate an automatic response: a habit.
The amount of time that it took for actions to become habits varied. Participants anywhere between 18 and 254 days to form a habit. The average number of days needed to achieve automaticity was 76 days.
Sleep in numbers
■ Two-thirds of adults in developed nations fail to obtain the nightly eight hours of sleep recommended by the World Health Organisation.
■ An adult sleeping only 6.75 hours a night would be predicted to live only to their early 60s without medical intervention.
■ A 2013 study reported that men who slept too little had a sperm count 29% lower than those who regularly get a full and restful night’s sleep.
■ If you drive a car when you have had less than five hours’ sleep, you are 4.3 times more likely to be involved in a crash. If you drive having had four hours, you are 11.5 times more likely to be involved in an accident.
■ A hot bath aids sleep not because it makes you warm, but because your dilated blood vessels radiate inner heat, and your core body temperature drops. To successfully initiate sleep, your core temperature needs to drop about 1C.
■ The time taken to reach physical exhaustion by athletes who obtain anything less than eight hours of sleep, and especially less than six hours, drops by 10-30%.
■ There are now more than 100 diagnosed sleep disorders, of which insomnia is the most common.
■ Morning types, who prefer to awake at or around dawn, make up about 40% of the population. Evening types, who prefer to go to bed late and wake up late, account for about 30%. The remaining 30% lie somewhere in between.
What are the origins of polyphasic sleep?
Some literature and research suggests that humans are naturally polyphasic sleepers. Avidan agrees that plenty of historical evidence points to biphasic sleep&mdashgoing to bed early, rising early, then napping for a prolonged period later in the day&mdashbut says he suspects the roots are more environmental than biological. “It was probably just related to environmental needs&mdashlack of air conditioning, and when it would be most comfortable to work in a hunter-gathering environment or agricultural environment,” he says.