Why do we sleep?
Sleep has been a part of the life of all organisms for 2.5 million years. Even individual cells take a break every 20 minutes, which could be considered as a first form of “sleep”. For humans, rest by sleep is essential, and permanent complete sleep deprivation leads to death. So, we sleep to stay alive! What is sleep then? Sleep is a very complex interplay of vital bodily functions. Without it humans cannot survive.
The foundation of our lives is the sleep-wake-cycle, and we are clearly creatures of habit in this respect. The basis is the homeostatic sleep drive and the circadian rhythm: the homeostatic sleep drive accumulates during wakefulness and promotes sleep. During sleep, it decreases again, the chemical correlate could be the neurotransmitter adenosine. The circadian rhythm (circa – about; dia – day) regulates the sleep-wake-rhythm approximately in a daily rhythm. It is genetically controlled on the one hand (CLOCK gene), and there is a pacemaker in the brain (more precisely, the hypothalamus). Last but not least, the circadian rhythm is synchronized by external time cues (daylight/darkness).
How do we fall asleep?
Darkness stimulates the pineal gland (epiphysis) to produce enough of the neurotransmitter melatonin, which promotes sleep. This occurs not only at night but also with a lack of light on dark winter days. The increased concentration of melatonin in the blood prepares us for sleep. Now, all systems switch to relaxation mode: The heart rate decreases, blood pressure and body temperature drop. Breathing becomes shallower and very regular. The muscles relax.
Our sleep has a rhythm!
During sleep, we go through a repeating cycle: Sleep onset phase → light sleep → deep sleep → REM sleep → light sleep → deep sleep → REM sleep, etc. Such a cycle lasts about 90 to 110 minutes and is repeated four to seven times per night. Especially the first two sleep cycles are important for brain recovery.
The so-called light sleep phase (stage N1) makes up for about 5% of total sleep time in middle-aged person (aged about 30 years), followed by stable sleep (N2) at about 45-55%. Deep sleep (N3) covers about 15-25%, and dream sleep (REM sleep) 20-25% of total sleep time. The latter is clearly defined by the well-known rapid eye movements.
Sleep exerts a strong influence on our body and well-being!
Healthy sleep promotes recovery and our overall well-being. On the opposite end of the spectrum, non-restorative sleep can turn our lives upside down because sleep can affect almost all bodily functions. Here are just a few examples: Sleeping for less than 6 hours and falling asleep after midnight are associated with an increase in body weight! (Obesity, Jorgensen 2015, 1 million study participants!). School children who sleep less than 9 hours per night also have a higher risk of obesity (van Cauter 2008). The hormone ghrelin makes us hungry, and with reduced sleep duration, ghrelin levels increase significantly, leading to a higher risk of obesity (Taheri 2004). The hormone leptin is produced in fat cells (adipocytes) and makes us feel full. Lack of sleep leads to a reduction in leptin levels and increased appetite (Hakim 2015).
Sleep also affects our intellect: Non-REM sleep promotes learning of facts, while REM sleep promotes mental recovery, maintaining concentration, procedural and emotional learning, and processing of daily events. Non-restorative sleep reduces our ability to perform everyday tasks: Sleep deprivation leads to increased sleepiness, as a consequence the risk for car accidents increases. Memory, attention, and concentration are reduced, and reaction time is significantly prolonged. In children, sleep deprivation is associated with a higher risk of hyperactivity.
The immune system is affected by sleep deprivation as well: Studies have clearly shown that cytokines are produced less during sleep deprivation, and that fewer antibodies are produced after flu vaccination in patients with sleep deprivation.
Finally, the hormone balance is negatively affected by non-restorative sleep. In addition to the hormones mentioned above in fat metabolism, hormones such as growth hormone, prolactin, and testosterone are also produced in lesser quantity.
So what is sleep good for? Almost everything!
And what if sleep is not normal?
Our sleep can be disturbed by many factors: The most important are sleep-related breathing disorders, in which harmful breathing pauses reduce oxygen saturation in the blood. But also so-called insomnia can make sleep non-restful due to stress.
What is sleep apnea?
Obstructive Sleep Apnea (OSA) describes repetitive collapses of the upper airway that can lead to partial or complete obstruction. These events often lead to a decrease in oxygen saturation in the blood (SaO2) and are usually terminated by short stress and arousal reactions of the body. The disease is common: 24% of men and 9% of women suffer from obstructive sleep apnea (OSA), 75% of OSA patients are still undiagnosed or untreated! Risk factors for sleep apnea are: increased body weight (obesity), anatomical reasons (enlarged tonsils, large soft palate, enlarged tongue base), hormonal (postmenopausal) in women, and also endocrine (hypothyroidism). Sleep apnea is also hereditary, so it runs in the family! The consumption of alcohol and sleeping pills also leads to increased apneas.
The main sites for airway obstruction are at the level of soft palate and uvula as well as the tongue base. In addition, all patterns of obstruction can occur in combination. Sleep apnea can lead to cognitive impairment, symptoms range from morning/daytime sleepiness to lethargy, to neuropsychological symptoms such as concentration impairment, reduced memory performance, depression, and irritability.
Due to the repeated breathing pauses and associated stress/arousal reactions, there can also be cardiovascular problems: sleep apnea is associated with coronary heart disease. Sleep apnea can be a direct trigger via oxidative stress and systemic inflammatory reaction with atherosclerosis. This can lead to a heart attack. Sleep apnea patients have a significantly increased risk of cardiac arrhythmias such as persistent atrial fibrillation, with an untreated risk of 82%, and a reduced risk of 42% with successful therapy. The same applies to the central nervous system: sleep apnea is an independent risk factor for stroke. In addition, sleep apnea is associated with a 2.5-3x increased risk of traffic accidents.
Diagnosis and Therapy
The exam includes extensive history taking and documentation with questionnaires, followed by clinical examination with endoscopy of the nose, throat, and larynx. Subsequently, in Germany, an examination with a so-called polygraphy device is required, which is a very small device that allows sleep diagnosis at home. This makes it possible to determine whether sleep apnea may be present and, consequently, whether further diagnostics in the sleep laboratory are necessary. If such diagnostics are required, they include a two-night stay in the sleep laboratory. The sleep laboratory enables precise diagnosis in a hotel-like environment. Brain waves and muscle movements are measured with electrodes, acoustic signals are evaluated with regard to snoring, and breathing is assessed using airflow sensors. The body position is registered as well. This makes it possible to localize the origin of apneas. In addition, other sleep disorders such as leg movements (restless legs syndrome, periodic limb movement disorder) can be diagnosed. Often, treatment can already be initiated during the same sleep laboratory stay, such as positive-pressure therapy or treatment with a positional therapy vest.
For less severe cases, treatment with a mandibular advancement splint may be possible. Surgery is also always considered as an alternative. This ranges from minimally invasive treatments using radiofrequency energy on the soft palate or tongue base to the reduction or removal of the tonsils, to therapy using a tongue stimulator (Hypoglossal Stimulator). To weigh these alternatives more precisely, endoscopic examination during a drug induced sleep can be useful: during a so-called sleep video endoscopy, the patient is examined during a drug induced sleep provided by the anesthesiologist. Endoscopic examination of the nose and upper airways is possible. This makes it possible to precisely locate the cause of the airway obstruction or snoring.
For other sleep disorders, collaboration with other medical disciplines (internal medicine, neurology) is also necessary.
