Since an average human spends one-third of their life asleep, it is apparent that the quality of sleep has an important impact on the overall life quality. Sleep has restorative power and improves human brain functioning, in particular memory and attention.
Sleepy people have problems with concentration and commit more mistakes, which can contribute to the fact that they are increasingly prone to accidents. What sleep really is and why is it so important? What are the night dreams? What factors influence their quality? What kind of disorders may it be subject to? We will try to answer the above questions.
Image credit: M. Osial
Sleep and sleep cycle
We all need to sleep. It is necessary for the proper functioning of our organism, in particular for our on-board computer, the brain [1]. Moreover, it helps us to ensure metabolic homeostasis [2]. However, insufficient sleep translates to our reaction, work effectiveness, learning capacity, and relations with other people.
Most of us require between 6-9 hours of sleep per day [3], while some scientific studies show that human natural sleep duration is close to 7 hours [4]. Some others suggest that on average, healthy adults need to sleep thirty percent of every day (7.2 hours) [5]. Newborns should sleep 14 to 17 hours, infants 12 to 15 hours, toddlers 11 to 14 hours, preschoolers 10 to 13 hours, school-age children 9 to 11, and teenagers 8 to 10 [6]. So, when we get older, we tend to sleep less. Even one bad night’s sleep decreases our alertness, attention span, concentration. Our problem-solving capabilities the next day are also worse.
How can we characterize sleep? Sleep should be treated as a process. It is a biological phenomenon, which has five stages following cyclically one after another (in healthy people) [7]. Phase 1 (non-rapid-eye movement, called NREM Stages 1) is the lightest called falling asleep and it lasts for 5-10 minutes.
The body quickly drifts in this phase, muscles relax, brain waves slow down, and at this stage, we can be awakened fairly easily. On average all NREM Stages 1 should last about 15% of whole sleep. Phase 2 (NREM Stages 2) is the light sleep and lasts about 20 minutes. All NREM Stages 2 should take 50% of the night's sleep. Here, everything slows down, including brain activity, heart rate, body temperature, and even breathing.
Phase 3 (NREM Stages 3) is the deepest sleep cycle in which the body repairs quicker than when we are awakened, and the body restores energy and strengthens the immunological system. When we summarize all NREM Stages 3 we should receive 30%.
The last phase is REM sleep (rapid-eye movement), and thanks to that phase we have dreams. During this phase, the brain is highly active and we start to dream, creating stories of fantastical, bizarre, and often illogical dreams. Here, also the mental revitalization begins. Surprisingly, this stage takes only 15% of sleep time. All these stages are repeated five to six times per night. Naturally, this cycle can be disrupted as a result of different kinds of sleep disorders.
What do you dream about?
Dreams have always fascinated mankind, and to a certain extent, they still do. We do not have many answers concerning why we dream, however, there are some theories [8]. One of them treats dreams as some sort of a repair mechanism, which enables the brain to recharge neural cells’ depleted stocks of transmitter chemicals.
Another theory, a philosophical one, created by Sigmund Freud, states that dreams are our repressed wishes [9]. In turn, a very widespread theory is that dreaming helps with memory consolidation, a time-dependent process in which recently acquired experiences are translated into long-term memory.
Nightmares may occur when negative emotions are processed. It is presumed that this happens by chemical and structural changes in the brain. Still, the exact mechanism behind it is unclear, but it may be because sleep allows us to process the information acquired while blocking out the stimuli that could potentially be distracting. There is research which links memory consolidation and therefore sleeps, with gaining insight. It was concluded that by reconstructing our memories we can extract knowledge and help with insightful behavior.
Remembering our dreams is also a very interesting topic. We all have dreams but we forget around 90% of the dreams that we have. The reason for this is somewhat complicated, but scientists believe that when we are asleep our brain waves change and this may result in us either recalling something or not.
It is believed that sometime during ‘alpha’ wave sleep, something stops us from waking up due to stimuli coming from the outside, however, people who tend to remember their dreams, wake up more often at night. That means that their alpha wave dip is not as large in comparison.
Well, what about lucid dreaming? It is an act of maintaining consciousness during the REM stage of sleep, allowing you to realize you are in a dream without waking up. This form of controlling our dreams can be used to somewhat manage nightmares, although not fully.
An experiment conducted by the Goethe University in Frankfurt showed that participants who were given electric zaps during their REM sleep phase claimed they experienced lucid dreaming right after. This works by introducing outside stimuli, reminding us that we are currently dreaming, but we do not need to be shocked by electricity to lucid dream. This can be practiced by keeping a dream journal and by reminding yourself you are awake, which might later help with recognizing dream states.
Sleep influence and importance
During daytime activities, our brains have irregular staccato beta waves, while when we sleep our brains are functioning on slower alpha and theta waves (see, Figure 1). In turn, when we are dreaming, our brain waves are more rapid and erratic just like when we are awake. Taking into account sleep stages, beta and alpha waves occur in the NREM Stage 1 and REM, theta waves in the NREM Stage 2 and REM, while delta waves in NREM Stage 3.
Figure 1. Wave types in the brain. Image credit: M. Osial
From the anatomical point of view, six different brain structures are involved in sleep processing, (see Figure 2). First, the hypothalamus is responsible for control centers affecting sleep and arousal. It also has nerve cells, which receive information about light exposure directly from the eyes, and in this way, control your behavioral rhythm.
The second one is the brain stem. It receives signals from the hypothalamus and controls the transitions between wake and sleep phases. It is also responsible for sending muscles and limb relaxing signals to our body. This makes the brainstem a very important brain part in the REM stage. Without it, we could not act out our dreams. In turn, thanks to the thalamus we are able to tune out the external world.
During most of the sleep phase, the thalamus is calm, the exception being REM, during which it passes on to the cortex images, sounds, and all components of our dreams. Another important brain part is called the pineal gland. It increases the production of the hormone melatonin, which helps us fall asleep when the light goes out.
The basal forebrain releases adenosine (a by-product of the energy used), which intensifies the feeling of sleepiness. It is possible to counteract this process by the caffeine that blocks adenosine. And the last one, the amygdala, which processes our emotion, is also more active in the REM stage.
Figure 2. Parts of the brain, which are involved in sleep. Image credit: M. Osial
Sleep is crucial to our restoring processes [10]. During sleep, our brain learns, i.e. stores new information and gets rid of toxic waste. Our body repairs cells restore energy, and releases molecules like hormones and proteins. It helps in regulating many important issues, including emotional health.
When we sleep, the activity of our brain increases in the areas which are responsible for emotions [11]. Sleep influences our weight by controlling the hunger hormones, such as ghrelin (appetite increment) and leptin (which increases the feeling of being full after eating). Untreated sleep disorders can also cause serious health problems and medical issues.
Summary
Humans are adapted to sleeping while it is dark outside. The body rests and regenerates during sleep, while the brain never stops working. The brain goes through a few phases during sleep, until the REM phase is achieved. Additionally, the particular phases can be disturbed with several disorders that affect the whole body. Sleep is important to our health and proper functioning, in particular for our brain, heart, and lungs to metabolism, immune functions, mood, and disease resistance.
This article is a joint work of Aleksandra Wójcik (Faculty of Chemistry, University of Warsaw), Karol Masztalerz (Faculty of Science and Engineering, University of Manchester), Agnieszka Pregowska (Institute of Fundamental Technology Research, Polish Academy of Sciences), and Magdalena Osial (Faculty of Chemistry, University of Warsaw) as a part of the Science Embassy project. Figures – M. Osial.
References:
[1] Wagner U, Gais S, Haider H, Verleger R, Born J. Sleep inspires insight. Nature. 2004;427(6972):352-5. DOI: 10.1038/nature02223. PMID: 14737168
[2] Xie L, Kang H, Xu Q, Chen MJ, Liao Y, Thiyagarajan M, O'Donnell J, Christensen DJ, Nicholson C, Iliff JJ, Takano T, Deane R, Nedergaard M. Sleep drives metabolite clearance from the adult brain. Science 2013;342(6156):373–7
[3] Hor H, Tafti M, How Much Sleep Do We Need? Science 2009, 325(5942):325–825
[4] Dijk DJ, Skeldon AC, BIOLOGICAL RHYTHMS Human sleep before the industrial era. Nature 2015;527: 176–177
[5] Klerman E, Dijk D, Age-related reduction in the maximal capacity for sleep – Implications for insomnia. Current Biology 2008;18:1118–23
[6] Hirshkowitz M, Whiton K, Albert SM, Alessi C, Bruni O, DonCarlos L, Hazen N, Herman J, Katz ES, Kheirandish-Gozal L, Neubauer DN, O'Donnell AE, Ohayon M, Peever J, Rawding R, Sachdeva RC, Setters B, Vitiello MV, Ware JC, Adams Hillard PJ. National Sleep Foundation's sleep time duration recommendations: methodology and results summary. Sleep Health. 2015;1(1):40-43. DOI: 10.1016/j.sleh.2014.12.010. Epub 2015 Jan 8. PMID: 29073412.
[7] Stickgold R, A memory boost while you sleep, Nature Neuroscience, 2006, 444(71119):559–560
[8] De Koninck, J. (2012). Sleep, dreams, and dreaming. In C. M. Morin & C. A. Espie (Eds.), Oxford library of psychology. The Oxford handbook of sleep and sleep disorders (p. 150–171). Oxford University Press. https://doi.org/10.1093/oxfordhb/9780195376203.013.0009
[9] Freud, Sigmund (2010). The interpretation of dreams. Strachey, James. New York: Basic Books A Member of the Perseus Books Group.
[10] Hobson JA. Sleep is of the brain, by the brain and for the brain. Nature. 2005;437(7063):1254-6. DOI: 10.1038/nature04283. PMID: 16251949.
[11] Yoo SS, Gujar N, Hu P, Jolesz FA, Walker MP. The human emotional brain without sleep–a prefrontal amygdala disconnect. Curr Biol. 2007;17(20): R877-8. DOI: 10.1016/j.cub.2007.08.007. PMID: 17956744.