Sleep stages & deep sleep - why each stage is important

A good night's sleep is much more than just "lying in bed for eight hours". Perhaps you know the situation: you have supposedly slept enough, but still feel tired and unrested in the morning. Why is that? The answer lies in the sleep phases. Our sleep is divided into different stages - from light dozing to deep dream sleep. Each phase fulfills specific tasks in the body and brain. If one of them is missing or falls short, our recovery suffers. In this article, we take a closer look at the stages of sleep and explain why **light sleep, deep sleep and REM sleep** are essential for your health. You'll also get practical tips on how to improve your sleep patterns and get more quality sleep - including restorative deep and REM sleep.

Sleep problems are common: It is estimated that around a quarter of the population complains of regular sleep disorders. Those affected are often unaware that it is not only the duration of sleep that plays a role, but also the **sleep architecture**. "Sleep architecture" refers to the sequence and proportions of the different sleep stages in a night. An adult normally goes through **4 to 6 sleep cycles** per night^[1]. Each cycle lasts around 90 minutes on average and contains all sleep stages - from light sleep to deep sleep and REM sleep. It is crucial that we get enough of each phase. Otherwise, we may not regenerate properly despite getting enough hours of sleep. In the following, we first explain which sleep phases there are and what happens in each of them. We then look at why each phase is important - for physical regeneration, memory, the immune system and more. And we'll give you helpful advice on how you can support your sleep so that all phases run optimally.

What are sleep phases?

Sleep is not a uniform state, but a **dynamic process** with changing stages. Experts roughly divide sleep into two categories: **REM sleep** (Rapid Eye Movement, the dream phase) and **Non-REM sleep** (non-REM sleep), which in turn is divided into several deep stages^[1]. In detail, a distinction is usually made between **four phases**:

• Phase 1 - falling asleep phase (N1): Transition from wakefulness to sleep. In this very light sleep stage, muscles relax and pulse rate begins to fall. You drift off, but are still easy to wake up.
• Phase 2 - light sleep (N2): The body comes more and more to rest. Pulse and breathing slow down, body temperature drops slightly. Special activity patterns appear in the brain (so-called sleep spindles and K-complexes), which help to filter out external stimuli^[3]. This phase accounts for around half of the total sleep time and prepares the body for deep sleep.
• Phase 3 - Deep sleep (N3): This is the deepest and most restorative sleep. The heartbeat and breathing are now at their slowest and the muscles are maximally relaxed. Slow delta waves appear on the EEG. It is very difficult to be woken up in this phase. Deep sleep is crucial for physical recovery and growth processes.
• Phase 4 - REM sleep: This phase is also known as dream sleep or paradoxical sleep. The brain is almost as active as when awake, the eyes move jerkily under the eyelids - hence the name Rapid Eye Movement. At the same time, the body muscles (except for the eyes and respiratory muscles) are paralyzed so that we do not act out our intense dreams. REM sleep occurs for the first time about 90 minutes after falling asleep and is then repeated several times.

A night consists of several such **sleep cycles** in which we go through the phases one after the other. At the beginning of the night, deep sleep dominates, while REM sleep becomes longer and longer towards the morning. A typical 90-minute cycle starts with phase 1, moves into phase 2, then into deep sleep and ends in a REM phase before the next cycle begins^[1]. Interesting: **Sleeping completely through the night without waking up is a myth.** Everyone has numerous brief waking moments each night - often at the end of a cycle - which are usually not remembered. These mini-waking phases are normal and are part of the sleep architecture. As long as you fall asleep again quickly afterwards, your sleep remains restful.

Overall, a healthy adult spends around **50-60% of the night in light sleep (phase N1/N2)**, around **20% in deep sleep (N3)** and around **20-25% in REM sleep**^[1]. However, these proportions are not fixed, but can vary from person to person^[11]. Factors such as age, stress, lack of sleep or alcohol influence the distribution of sleep phases. For example, **newborns and children** have relatively more deep and REM sleep, while **older people** have significantly less deep sleep and wake up more frequently^[1]. The interaction of all stages is important: each stage fulfills certain functions, which we will now discuss in more detail.

Light sleep (stages N1 & N2): The gateway to deep sleep

The **light sleep stages** N1 and N2 are often underestimated. You might think: "Oh, what's the point of light sleep, the important thing is deep sleep." However, light sleep actually prepares our body and mind to enter deep sleep in the first place. It is not physiologically possible to fall into deep sleep directly from the waking state - we first have to go through the "pre-sleep" stages.

In phase N1, the falling asleep phase, our consciousness begins to switch off. We doze off, thoughts flicker briefly and then disappear. Sometimes the muscles twitch suddenly in this phase (hypnic jerks) - a sign that the brain is initiating the transition to sleep. This phase usually only lasts a few minutes. It is like shallow water on the beach: you slowly feel your way from dry (awake) to deeper water.

Phase N2, the actual light sleep, takes up by far the largest part of our sleep - roughly half of the night. Here the body relaxes further: eye movements stop, muscle tension decreases, heartbeat and breathing become even more regular and slower. Characteristic **sleep spindles** can be measured in the brain - these are short bursts of rapid brain wave activity. These spindles have an important protective function: they shield us from external disturbances. Imagine a car driving past outside at night - in light sleep, sleep spindles prevent the brain from reacting to this so that we don't wake up straight away. Interestingly, sleep spindles are also associated with **memory processes**: Studies suggest that they may be involved in the consolidation of new memories^[3]. The more sleep spindles someone has, the better they sometimes perform in learning tests - an indication that even inconspicuous light sleep has cognitive significance (however, sleep spindles are only one piece of the puzzle, further research is ongoing).

Without sufficient light sleep, we would hardly find our way into deep sleep. Phase N2 acts as a **transition bridge**: Body functions shut down enough to initiate the next step - deep sleep. It is also interesting to note that short naps (power naps) usually only lead to light sleep. A nap of 20 minutes, for example, keeps us in phase 2 and refreshes us without us falling into deep sleep - so we wake up again relatively easily. This shows that light sleep definitely has a restorative value, especially mentally. It's not for nothing that you often feel clearer in the head after a light doze.

Important to know: **Light sleep is susceptible to disturbances: noises, light or movement are most likely to cause awakening during these phases. So if you wake up frequently in the first half of the night, this is often because you were still in phase 2. Good sleep hygiene (quiet, dark bedroom, little noise) helps enormously here to glide undisturbed through the light sleep phases into deep sleep. All in all, it can be said that light sleep sets the stage for the following stages - deep sleep and REM sleep - to develop their full potential.

Deep sleep (phase N3): Nightly regeneration at full speed

**When we talk about "a good night's sleep", many people are referring primarily to deep sleep. And indeed, phase N3, deep sleep, is essential for physical recovery. During this phase, numerous repair and rebuilding processes in the body run at full speed. Deep sleep can be thought of as the body's nocturnal workshop, in which damage from the day is repaired and supplies are replenished.

Deep sleep is characterized by the occurrence of **delta waves** in the brain, very slow and high-amplitude brain waves. They indicate that the nerve cells are firing in a synchronized manner and the brain switches to a state of low activity. At the same time, the body is in an **energy-saving mode**: Body temperature drops slightly, respiratory rate and pulse are at their lowest levels during the night. We lie there really "deep asleep" - it is extremely difficult to wake someone in deep sleep. If it does happen (e.g. due to a loud bang), you are initially completely disoriented and need some time to wake up.

What happens during deep sleep? A whole lot of good things:

• The release of **growth hormone (GH)** peaks. Around 70% of the daily growth hormone is released during deep sleep. This hormone is not only important for growth in children, but is also essential for adults - it supports cell renewal, wound healing, muscle building and the breakdown of fat reserves. In short: cell repair and regeneration take place on a large scale during deep sleep ^[4].
• The **immune system** runs at full speed. Research shows that deep sleep strengthens the function of our immune system. During deep sleep, certain immune cells (such as natural killer cells) become more active and inflammatory substances are regulated. Conversely, a lack of sleep - especially a lack of deep sleep - leads to increased levels of inflammation in the body. People who sleep chronically poorly are more susceptible to infections: In one study, those who slept less than 6 hours were around four times more likely to catch a cold than those who slept 7+ hours. These results show how important deep sleep is for a robust immune defense ^[5][6].
• The brain uses deep sleep for **"cleansing "**: Scientists have discovered a special disposal system in the brain - the glymphatic system. Put simply, during deep sleep, more cerebrospinal fluid is flushed through the brain tissue to remove waste products such as beta-amyloid (a protein linked to Alzheimer's disease). A high-profile mouse study in 2013 found that the brain removes waste products significantly faster during sleep than when awake. Deep sleep could therefore help to "clear the brain" and prevent neurodegenerative diseases - a fascinating field of research (note: this glymphatic theory is currently being further researched and discussed) ^[7].
• The **blood pressure** drops significantly during deep sleep. This relieves the cardiovascular system. People who do not get enough deep sleep (e.g. due to sleep disorders or apnoea) are more likely to have blood pressure problems. Deep sleep therefore has the effect of naturally lowering blood pressure at night and is therefore important for heart health.
• **Memory and learning** also benefit from deep sleep. Although a lot of memory processing also takes place during REM sleep (more on this in a moment), deep sleep seems to be particularly important for reinforcing newly learned facts and motor processes. During deep sleep, memories are transferred from the hippocampus (a kind of buffer) to the cerebrum and consolidated there. This process is called consolidation. Studies show, for example, that test subjects can remember what they have learned better if they have had a night with sufficient deep sleep after learning. Deep sleep is therefore the phase in which our brain sorts and permanently stores what we have learned during the day ^[2].

Given this list, it is not surprising that a lack of deep sleep makes us feel physically exhausted and susceptible to illness. Even a single sleepless night, for example, drastically reduces the number of immune cells in the blood the next day. If deep sleep is missing for a longer period of time, the hormonal balance and metabolism are thrown out of sync - more stress hormones are released and fewer appetite-regulating hormones, which can contribute to weight gain in the long term. Skin regeneration also suffers (keyword "beauty sleep" - it actually exists!). Deep sleep is therefore the basis for physical vitality.

Good to know: The majority of deep sleep occurs in the first one or two sleep cycles of the night, i.e. approximately the first 3-4 hours of sleep. After that, the duration of deep sleep per cycle decreases. This explains, for example, why **shift workers** who split their sleep or sleep at unusual times are often less rested - they lack the undisturbed deep sleep in the natural first third of the night. The same applies to **alcohol consumption:** Alcohol allows you to fall asleep "deeply" quickly, but it disrupts the sleep architecture. The deep sleep phases come too early and are of lower quality, followed later by an unfavorable REM rebound with restless sleep. You then often wake up early and feel exhausted. So the rule is: make sure you get enough and good deep sleep - it is the basis for waking up physically refreshed in the morning.

REM sleep: dreaming for the mind & psyche

After around 90 minutes of sleep, the **REM sleep phase** occurs for the first time - and with it a completely different world of nocturnal activity. In the REM (Rapid Eye Movement) phase, the brain is highly active; EEG measurements show a pattern that is similar to that in the waking state. At the same time, the body is paralyzed (atonic) - a protective mechanism to prevent us from acting out our vivid dreams. This combination of a sleeping body and an awake brain led to the term "paradoxical sleep" for the REM phase.

**REM sleep is the phase of intense dreams. Of course, you can dream in other phases, but REM dreams tend to be the most vivid, surreal and memorable. In a single night, we go through four to six REM phases, which can last from just a few minutes at the beginning to almost an hour towards morning. In total, an adult spends about a quarter of the night in REM sleep. During this time, our brain does amazing things on a mental level:

• Memory and learning: while deep sleep tends to consolidate facts and "hard knowledge", REM sleep promotes **linking and getting creative** with knowledge. Studies have shown that REM sleep primarily helps with procedural learning (e.g. learning skills) and creative problem solving. In one experiment, test subjects had to form word associations; those with sufficient REM sleep made significantly more unusual and creative connections. REM therefore seems to allow the brain to think "out of the box" - networks are associated more freely. Insights into previously learned tasks often only occur after sleep (including REM). The saying "sleep on it" has a scientific core here ^[8].
• Emotional processing: REM sleep is associated with the regulation of our emotions. It apparently helps us to categorize emotional experiences and reduce their **intensity**. There is evidence that after a night of normal REM sleep, stressful events have less of a stress-inducing effect - the brain has "digested" the emotion in the dream, so to speak. Neurobiologically interesting: In REM phases, the amygdala (emotion center) and the hippocampus (memory) are active, while the prefrontal cortex (control of reason) is more subdued. This could explain why dreams are often emotional and bizarre. By repeatedly confronting emotions in dreams (in a safe environment, as we sleep), we apparently learn to react more calmly the next day. An example: Someone who has experienced something sad usually feels better after "sleeping on it for a night" - the memories are still there, but the emotional pain is somewhat alleviated. Research supports this: REM sleep can help to consolidate emotional memories without retaining the full stressful charge ^[9].
• Mood and psyche: Sufficient REM sleep appears to be important for a stable mood. If people are deprived of REM sleep (while deep sleep is maintained), they often become irritable, anxious or depressed. Interestingly, REM abnormalities often occur in depression (e.g. shortened REM latency, i.e. falling into REM too early). Chronic lack of REM sleep - for example due to sleep disorders - can therefore impair mental health. For example, a large meta-analysis shows that untreated insomnia significantly increases the risk of depression ^[10]. Good sleep with sufficient REM phases, on the other hand, acts like a nightly "therapy session" in which the soul is allowed to sort itself out.
• Physical functions in REM: In REM sleep, the body is almost motionless, but certain processes are running internally: Blood pressure and pulse fluctuate more, as if we were undergoing small "stress test runs" - this keeps the cardiovascular system flexible. Breathing can also be irregular. Incidentally, male sleepers experience regular erections during REM (similar to blood flow to the clitoris in women) - a normal physiological phenomenon that has nothing to do with the dream content. This blood flow keeps the tissue healthy. Overall, REM sleep is therefore by no means "lazy", but an active phase that keeps the body on its toes.

Because so much happens in the brain during REM sleep, some researchers refer to it as **"mental cleaning staff "**. Memory content is reorganized, emotions are classified and creativity may also be encouraged. It is not for nothing that artists and inventors often report that an idea came to them "in a dream". In fact, there are historical anecdotes (e.g. Dmitri Mendeleev and the periodic table) where a problem was solved in a dream. REM sleep therefore provides mental freshness and emotional balance.

Fun fact: The duration of REM sleep decreases over the lifespan. **Babies** spend up to 50% of their sleep time in REM - this is thought to aid rapid brain development. In **adults** it is ~25 % as mentioned above. With **age**, the proportion of REM sleep may decrease slightly, but less so than deep sleep. Much more important, however, is the overall quality of sleep: older people are more likely to suffer from fragmentation (frequent waking), which chops up both deep and REM sleep. So when Grandma Erna says "I hardly ever dream anymore", it's often because she generally has less continuous sleep. Conversely, it turns out that with enough sleep, even seniors can still have intense dreams - the ability to dream is retained.

Keeping sleep phases in balance: How to optimize your sleep

We now know that restful sleep consists of a **harmonious interplay of all phases**. How can we promote this balance? The good news is that you can do a lot to help your body go through all sleep stages undisturbed and at the right time. Here are some specific tips for optimizing sleep - many of which are aimed at improving your sleep rhythm and sleep quality so that light, deep and REM sleep take place at healthy levels.

• Consistent sleep-wake rhythm: Our body follows an internal clock (the circadian rhythm). If you go to bed and get up at roughly the same time every day, your body can adjust to this optimally. Regular timing particularly promotes the occurrence of deep sleep in the first cycles and sufficient REM in the morning hours, because your body knows when it is time to go to sleep. Therefore, try not to turn your rhythm completely upside down, even at the weekend. Small deviations are okay - but don't pull an all-nighter and only go to bed at 5 a.m., for example, as this upsets your internal clock.
• Sleep hygiene & sleeping environment: Ensure a sleep-friendly environment. A dark, quiet and cool bedroom helps you to fall asleep and stay asleep. In complete darkness, the pineal gland releases more melatonin - a hormone that induces sleep. Noise should be minimized as much as possible (earplugs or white noise can help if necessary). The optimum temperature is around 16-19 °C - it is easier for the body to enter deep sleep in cooler surroundings. And: If possible, only use your bed for sleeping. If you work or watch TV in bed, your brain associates the bed with activity rather than relaxation. A clear bed=sleep association helps you to switch off more quickly and shorten the transition phases (N1/N2).
• Evening routine and relaxation: Introduce an evening ritual to help you wind down. For example: a warm shower or bath (this lowers your body temperature, which promotes sleep), soft music, reading or light stretching exercises. Meditation or breathing exercises before sleep lower stress hormones and allow you to drift gently off to sleep. It is important to avoid **screen time** in the last hour before going to bed - the blue light from cell phones, tablets, etc. inhibits melatonin production and makes it harder to immerse yourself in phase 1. Better to do something analog and relaxing. With a good routine, you signal to your body: "Bedtime is approaching" - this makes it easier to fall asleep and lets you sleep more deeply.
• Diet and stimulants: Avoid heavy meals late at night as well as caffeinated drinks. Caffeine can delay the sleep stages or prevent you from falling asleep because it blocks the adenosine receptors (adenosine is a drowsiness inducer). Ideally, therefore, don't drink coffee or energy drinks after 3 pm. Alcohol is also counterproductive: it may help you fall asleep, but it disrupts the sleep phases later on, especially REM sleep and deep sleep. The result: unrested sleep despite an initially deep slumber. If you are hungry in the evening, reach for a light snack such as a banana, yogurt or a handful of nuts - nothing that will make your stomach and intestines work overtime.
• Exercise during the day: Regular physical activity has been proven to improve sleep. Those who work off energy during the day (whether through sport or just brisk walks) build up more "sleep pressure". In particular, exercise promotes deep sleep the following night. Caution: Intensive workouts should not be done directly before going to bed, as they have a short-term activating effect. It is better to exercise in the late afternoon or early evening - this ensures a pleasant physical tiredness a few hours later.
• Reduce stress and calm your thoughts: Ruminating is an enemy of sleep, especially in the early stages. If worrying makes it difficult to fall asleep, try strategies such as journaling (transferring thoughts from your head to paper), progressive muscle relaxation or guided sleep meditation. The calmer your mind is when you fall asleep, the more undisturbed you will glide through phases 1 and 2 into deep sleep - without waking up at night. Mental relaxation = stable sleep phases.
• Use sleep trackers with caution: If you're curious about your sleep phases, you can try a fitness tracker or sleep app. These devices use movement and heart rate to estimate when you were in which phase. The data is interesting, but not 100% accurate. So don't be put off if the tracker shows "only 30 minutes of deep sleep" - the important thing is how rested you feel. However, trackers can help you to recognize patterns (e.g. "less REM sleep after alcohol"). Use the information as a rough guide, but above all listen to your body.
• Food supplements and aids: In persistent cases or special situations, sleep aids can also be useful. **Melatonin**, for example, is popular as a supplement to regulate the sleep-wake rhythm - officially, melatonin in a dosage of 1 mg helps to shorten the time it takes to fall asleep (approved health claim in the EU). This can be helpful for shift workers or those suffering from jet lag. Other natural sleep aids are herbs such as valerian, hops, passionflower or lavender. These traditional sedatives can have a subjectively relaxing effect, but do not have a health claim confirmed by the EFSA (not yet sufficiently evidence-based) *Not yet confirmed by the EFSA; further studies required* You can find such plants in some tea blends or preparations, for example. Minerals such as magnesium (for normal nerve and muscle function) or glycine (an amino acid that can have sleep-promoting effects) are also often taken in the evening. * The links between glycine and sleep have not yet been confirmed by the EFSA; further studies are required.* Important: These supplements are at best supportive - they do not replace good sleep hygiene. If you want to try dietary supplements, stick to the recommended dosage and speak to a doctor beforehand if you have any existing illnesses. Incidentally, at XTRAFUEL we offer a specially developed sleep complex that combines melatonin with herbal extracts - as a gentle aid for your evening routine.

To summarize: With a regulated lifestyle, a good sleep environment and a few tricks, you can do a lot to ensure that your body runs through all sleep phases optimally. Many of the tips above are interlinked - e.g. exercise during the day supports the night, relaxation in the evening promotes a quick drift into deep sleep, regular times stabilize your internal clock. Try out which measures are most beneficial for you personally. You will often notice significant improvements after just one or two weeks of consistent implementation - you will wake up more refreshed, have fewer deep sleep deficits (recognizable, for example, by the fact that you no longer feel exhausted in the morning), and dream recall can also become more intense when REM sleep is undisturbed. You can find more detailed tips on sleep optimization in our guide - take a look there to deepen your sleep knowledge.

Conclusion: every sleep cycle counts

Sleep is complex - but that is precisely where its strength lies. Our nights are orchestrated in **phases** that build on each other: Light sleep gently guides us into the depths, deep sleep repairs our body, and REM sleep takes care of our soul and memory. None of these phases is "superfluous". Anyone who only focuses on deep sleep and thinks that the dream phase is unimportant is not doing themselves any favors. It would also be fatal to think that you can do without deep sleep as long as you dream enough. **Good sleep is like a symphony with different movements - only all of them together create harmony.

Science impressively confirms how closely sleep phases and health are linked: Poor or too little sleep increases the risk of all kinds of problems - from infections and mood swings to concentration problems. Conversely, restful sleep is a real fountain of health: The immune system remains strong, the brain efficient and the mood stable. It is therefore worth making sleep a high priority. Don't see your bedtime as "lost time", but as an **investment in yourself**. In the hours you sleep, you are working on your health and performance for the next day.

If you are currently suffering from sleep problems, don't despair. Simple changes - as described above - can often have a big impact. Be patient with yourself: sleep is a sensitive system that improves gradually. Give your body the chance to get used to new routines. And if, despite all the measures taken, serious sleep disorders persist for a long time (e.g. persistent insomnia, suspected sleep apnoea, etc.), don't hesitate to seek professional help. Sleep doctors can specifically analyze which phase of your sleep is out of sync and work with you to find ways to restore balance.

Finally, an encouraging message: **Your sleep means you no harm - it wants to help you.** By giving it what it needs (rest, regularity, a little nurturing), you'll be rewarded with better nights and more alert, energized days. Every stage of sleep you go through is like a small win for your body and mind. With this in mind: consciously treat yourself to all the stages again tonight - and wake up refreshed tomorrow, ready to tackle your day!

Disclaimer: This article provides an overview of sleep phases and possible measures to improve sleep. Everyone is different - not all of the tips mentioned will work the same for everyone. Health-related statements on food supplements or herbs have not been confirmed by the European Food Safety Authority, unless explicitly labeled as EFSA-tested; in case of doubt, further scientific studies must be awaited. This article does not replace medical advice. Medical advice should be sought if you have severe or chronic sleep problems.

Sources:

1. [1] Patel, A.K., Reddy, V., & Araujo, J.F. (2022). Physiology, Sleep Stages. In: StatPearls [Internet]. StatPearls Publishing. URL: https://www.ncbi.nlm.nih.gov/books/NBK526132/
2. [2] Diekelmann, S., & Born, J. (2010). The memory function of sleep. Nature Reviews Neuroscience, 11(2), 114-126. DOI: 10.1038/nrn2762
3. [3] Schönauer, M., & Pöhlchen, D. (2018). Sleep spindles. Current Biology, 28(19), R1129-R1130. DOI: 10.1016/j.cub.2018.07.035
4. [4] Van Cauter, E., Leproult, R., & Plat, L. (2000). Age-related changes in slow wave sleep and REM sleep and relationship with growth hormone and cortisol levels in healthy men. JAMA, 284(7), 861-868. DOI: 10.1001/jama.284.7.861
5. [5] Irwin, M.R., Olmstead, R., & Carroll, J.E. (2016). Sleep disturbance, sleep duration, and inflammation: a systematic review and meta-analysis of cohort studies and experimental sleep deprivation. Biological Psychiatry, 80(1), 40-52. DOI: 10.1016/j.biopsych.2015.05.014
6. [6] Prather, A.A., Janicki-Deverts, D., Hall, M.H., & Cohen, S. (2015). Behaviorally assessed sleep and susceptibility to the common cold. Sleep, 38(9), 1353-1359. DOI: 10.5665/sleep.4968
7. [7] Xie, L., Kang, H., Xu, Q., Chen, M.J., Liao, Y., Thiyagarajan, M., et al. (2013). Sleep drives metabolite clearance from the adult brain. Science, 342(6156), 373-377. DOI: 10.1126/science.1241224
8. [8] Cai, D.J., Mednick, S.A., Harrison, E.M., Kanady, J.C., & Mednick, S.C. (2009). REM, not incubation, improves creativity by priming associative networks. Proceedings of the National Academy of Sciences (PNAS), 106(25), 10130-10134. DOI: 10.1073/pnas.0900271106
9. [9] Hutchison, I.C., & Rathore, S. (2015). The role of REM sleep theta activity in emotional memory. Frontiers in Psychology, 6, 1439. DOI: 10.3389/fpsyg.2015.01439
10. [10] Baglioni, C., Battagliese, G., Feige, B., Spiegelhalder, K., et al. (2011). Insomnia as a predictor of depression: a meta-analytic evaluation of longitudinal epidemiological studies. Journal of Affective Disorders, 135(1-3), 10-19. DOI: 10.1016/j.jad.2011.01.011
11. [11] Yetton, B.D., McDevitt, E.A., Cellini, N., Shelton, C., & Mednick, S.C. (2018). Quantifying sleep architecture dynamics and individual differences using big data and Bayesian networks. PLoS ONE, 13(4), e0194604. DOI: 10.1371/journal.pone.0194604