Sleep is a behavioral and physiological state of rest that animals have acquired through their long evolutionary history to efficiently and successfully adapt to changes in the internal biological and external terrestrial environments. Until the middle of the 20th century, mammalian sleep was regarded as neural and physical hypofunction resulting from exhaustion, while neuroscientific progress since the discovery of REM sleep has revealed that it is an actively generated condition consisting of REM and non-REM periods, which are controlled through statespecific sleep centers and networks in the brain. In the present article, the author reviews current advances in physiological aspects of human sleep and describes three major components that cooperatively control sleep amount, timing, and occurrence, with a focus on sleep homeostatic, circadian, and wake-maintenance systems.

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This paper reviews some of the representative epidemiological evidence for the relationship between sleep and physical and mental health. Sleep has elements of quantity (sleep duration), quality, and rhythm, and the evidence is summarized by focusing on abnormalities of each element. Regarding sleep duration, we present the results of a systematic review and meta-analysis of short sleep duration and health outcomes. Similarly, the results of a study on longer sleep duration and health outcomes are also presented. Regarding abnormalities in sleep rhythms, a systematic review and meta-analysis of systematic reviews and meta-analyses conducted on shift work and various health outcomes are reviewed to examine the evidence. The relationship between sleep and mental health is discussed in terms of the relationship between sleep and depression in previous epidemiological studies. Based on the findings from these observational epidemiological studies, future research is expected to be based on sleep hygiene education and intervention studies aimed at improving health by improving sleep.

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Insomnia symptoms (difficulty initiating sleep, difficulty maintaining sleep, and early morning awakening) are common, with a prevalence of 20% in the Japanese general adult population. In this paper, the author describes the clinical steps in the differential diagnosis of insomnia symptoms for readers who are not specialized in sleep medicine. When patients complain of insomnia symptoms, physicians first need to check their sleep habits and optimize them if they are inappropriate and interfere with good sleep. Medical history and medication use should also be reviewed in the first step because some medical conditions (e.g., articular diseases, skin diseases, cardiopulmonary disorders) and medications (e.g., steroids, interferon, methyldopa, levodopa) can cause insomnia symptoms. Insomnia symptoms can be caused by a variety of sleep disorders other than insomnia disorders, such as obstructive sleep apnea, restless legs syndrome, periodic limb movement disorder, and delayed sleep-wake phase disorder. A diagnosis of insomnia disorder is given only if the insomnia symptoms cannot be fully explained by another sleep disorder.

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Insomnia is a common symptom encountered by various specialists in their daily clinical practice. As a result, it is not uncommon for doctors to prescribe benzodiazepine (BZD) sleeping pills. However, the toxicity and adverse effects of BZDs have received much public attention, and the Japanese government has revised the fees for healthcare services in relation to the prescription of psychotropic drugs in an effort to reduce the problems of mass BZD prescription, polypharmacy of BZDs and long-term BZD use in Japan. This review attempts to explain the non-pharmacotherapeutic, pharmacotherapeutic and other features of drug withdrawal based on guidelines for the proper use of sleeping pills.

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Sleep-related disorders, especially in teenagers, have been receiving increasing attention. Here, we describe the current situation regarding sleep disorders and the habituation of sleep in teenagers, as well as the relationship between neurodevelopmental and sleep disorders. In teenagers, habituation of sleep, which includes eveningness, eveningness-related sleep insufficiency, and daytime sleepiness, has been reported to be associated with the increased prevalence of the two-income family, the influence of parents’ lifestyles, and excessive screen time on computers and mobile devices. The rapidly rising Internet utilization rate is related to increased screen time, which promotes delayed bedtimes, especially among primary schoolchildren. It has been reported that the more frequently Japanese low-teens go to sleep at a delayed bedtime, the more readily mental health disorders can develop. According to the International Classification of Sleep Disorders-Third Edition, sleep disorders in young adults contribute to restless legs syndrome, hypersomnia, and circadian rhythm sleep–wake disorders. It is difficult to diagnose restless legs syndrome in small children because their communication is limited. Hypersomnia has been shown to contribute to narcolepsy and idiopathic hypersomnia. Generally, idiopathic hypersomnia involves less frequent sleep-onset rapid eye movement (REM) periods than narcolepsy, as estimated by polysomnography and multiple sleep latency tests. Although narcolepsy includes REM-related symptoms such as catalepsy and sleep paralysis, idiopathic hypersomnia is diagnosed without REM-related symptoms. Delayed sleep–wake phase disorder, which is considered a circadian rhythm sleep–wake disorder, is often found in teenagers and can easily lead to diminished social life activity because it causes extreme difficulty awakening in the morning. In terms of neurodevelopmental disorders, sleep disorders such as restless legs syndrome, nocturnal enuresis, and sleep apnea syndrome are often said to complicate children with attention-deficit/hyperactivity disorder. Therefore, general practitioners and hospital doctors should refer teenagers likely to be experiencing hypersomnia or neurodevelopmental disorders to specialized institutions because most of these cases require early medical intervention.

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Sleep disorders are frequently observed in 40–60% of dementia cases. The complications of sleep disorders cause a decrease in the quality of life of patients and a burden on caregivers. It is clinically very important to address sleep disorders because they can be a factor in the worsening of behavioral and psychological symptoms of dementia. Sleep disorders in those with dementia are often caused by a combination of various factors and have complex pathologies, so they are difficult to treat. In this paper, we classify the factors associated with sleep disorders in dementia into three categories and describe how to address them in clinical practice.

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With the outbreak of the coronavirus (COVID-19), lifestyles have changed in Japan, which has affected sleep quality. Poor sleep quality can lead to physical illness, but sleep apnea in particular has been associated with many complications. Therefore, early detection and early prevention are important. Among sleep-related respiratory disorders, obstructive sleep apnea syndrome and obesity hypoventilation syndrome are diseases with high cardiovascular risk and are therefore outlined here. We also mention medical examination methods, diagnostic methods, treatment methods, and future prospects for sleep-related respiratory disorders in general outpatient clinics.

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Sleep-disordered breathing (SDB) is considered to be closely related to lifestyle diseases such as heart disease, hypertension, and diabetes. Its strong relationship with various cardiovascular (CV) disorders has recently become recognized in Japan. The diagnosis and treatment of SDB appears to affect the prognosis of CV disorders. Accordingly, doctors specializing in CV disorders need to be involved in the management of SDB. However, the level of involvement has not been sufficient to date. The author(s) conducted coronary bypass surgery on a patient, but the postoperative decline in heart function persisted. Upon in-depth examination, SDB was revealed. For this patient, continuous positive airway pressure drastically improved the ejection fraction from 40% to 65%. Since then, we have used a portable sleep Polygram instrument to screen heart surgery patients (1,005 patients) for the first time worldwide (NU-SLEEP study). SDB was observed in 77% of the patients. Among them, 16% of patients had severe SDB. These results indicate a strong association between CV disorders and SDB. In this featured article on SDB, the author(s) summarizes the involvement of SDB in various CV disorders.

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A variety of factors can contribute to nocturia, including polyuria, nocturnal polyuria, bladder storage disorder (benign prostatic hyperplasia, overactive bladder, etc.) and sleep disorders. Based on clinical guidelines for nocturia published by the Japanese Continence Society and Japanese Urological Association, the diagnosis and treatment of nocturia associated with sleep disorders are reviewed.

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