Normal Sleep, Sleep Restriction and Health Consequences
Government publications, such as 'Healthy People 2000' and its sequel 'Healthy People 2010', contain recommendations for adequate nutrition and physical fitness for healthy functioning, but no recommendations or standards for "normal" sleep duration and quality. Epidemiological data have never been obtained defining normal sleep and wakefulness as measured systematically by both subjective and objective indicators in infants, children, adolescents, young adults, middle-aged and older adults. Only limited EEG sleep data as a function of age and gender are available from laboratory studies published more than 25 years ago. Data used to describe 'normal' EEG sleep from infancy to old age were based on one or two nights of sleep recordings in a small number of subjects in a laboratory setting. Most of these studies were conducted prior to the establishment of accepted sleep monitoring and scoring standards. In fact, the most widely used reference of 'normal' human EEG sleep is based on studies in which EMG recordings were not used in the scoring of REM sleep.
Despite beliefs about the importance of sleep for health and normal growth and development, there are no standards of sleep physiology based on current polysomnographic criteria. Furthermore, there is no comprehensive database defining normal sleep-wake behavioral patterns by age or sex across the life span. Thus, health care providers have no normative reference for comparison with an individual's sleep pattern alone or as it relates to good health, and public health agencies have no way of knowing whether there are population shifts in the quality and quantity of sleep being obtained by different age groups.
Descriptions of sleep phenotypes and definitions of normal sleep patterns and requirements must incorporate the wide range of normal developmental and physical maturational changes across the life span. Although cross-sectional studies yield important information regarding sleep in discrete age groups, they do not address the evolution and persistence of sleep/wake patterns across time. There is a need to understand the complex reciprocal relationship between sleep and cognitive/emotional development from the prenatal period through adolescence and through adulthood. Prospective longitudinal studies utilizing validated screening and assessment tools are thus needed to delineate the development of sleep patterns and behaviors and to generate predictive models.
Progress In The Last 5 Years Sleep and Environment:
- Sensory stimulation resulting from environmental noise, light, motion, temperature, and even odors can produce activation at levels antithetical to the initiation and maintenance of sleep. Thus, sleep environments in which sensory stimulation is minimal (e.g., dark, quiet, comfortable temperature) tend to be preferred subjectively and tend to enhance sleep initiation and maintenance.
- Recent efforts by some hotels to offer rooms that are especially conducive to sleep, and by some transportation industries to improve the sleep environments of both customers and employees, reflect recognition of the importance of sleep and a sleep-conducive environment.
- Despite knowledge of the importance of environmental variables for sleep quality and duration and of considerable variations in sleep environments across ethnic and socioeconomic strata, there has been little scientific investigation of environmental factors potentially critical for healthy sleep and waking. Subjects requiring scientific investigation include:
Co-sleeping or bed-sharing (sleeping with one or more other persons in the bed): Although evidence suggests that the somesthetic stimulation resulting from bed-sharing can have a negative effect on sleep continuity and architecture, the relative benefits and adverse consequences of this practice have not been adequately explored.
Sleep location: Sleep at home versus sleep in public places in which control over environmental stimulation is minimal (e.g., on an airplane), or in the work environment (e.g., sleeper berths on trucks, trains and planes).
Sleep position: This has been determined to be important in Sudden Infant Death Syndrome (SIDS) and in modulating the severity of Sleep-Disordered Breathing (SDB), but sleep position has not been studied relative to the potential for sleep fragmentation (e.g., sleeping semi-recumbent vs. fully recumbent).
Sleep surface: Virtually ignored in sleep science, many consider optimal sleep surface to be crucial to obtaining a good night's sleep.
Environmental noise/light: Ambient noise and vibration have occasionally been studied as factors improving or impairing sleep quality, but little is known regarding the extent to which various types of noise and vibration affect sleep. A few experiments have found that light at night can enhance alertness, and that properly timed light exposure can hasten phase shifts of circadian biology (e.g., shift workers). There have been no studies, however, to determine whether turning on room lights at night adversely affects sleep quality or quantity.
Health disparities and vulnerable populations: Socioeconomically disadvantaged populations may be more likely to sleep in environments that are hot, humid, cold, noisy, and/or crowded. There has been no systematic study of the effects of these factors on sleep quality and quantity either alone or in combination, and no study of potential effects of these factors on waking functions including school and work performance. Children and adolescents, the elderly, and shift workers are populations who may be especially vulnerable to the adverse effects of environmental variables on sleep quantity and quality.
It is important to determine the extent to which each of these environmental factors affect sleep quality and contribute to sleep loss, sleep pathology, daytime sleepiness, and daytime functioning. Scientific data in these areas would be fundamental to answer questions regarding the determinants of a good night's (or day's) sleep.
Health Consequences of Insufficient Sleep and Chronic Sleep Debt:
- Adequate sleep is essential for healthy functioning and survival. Inadequate sleep and unhealthy sleep practices are common, however, especially among adolescents and young adults. In the 2002 National Sleep Foundation annual survey, nearly 40% of adults 30 to 64 years old, and 44% of young adults 18 to 29 years old reported that daytime sleepiness is so severe that it interferes with work and social functioning at least a few days each month. Excessive daytime sleepiness is a major public health problem associated with interference with daily activities including cognitive problems, motor vehicle crashes (especially at night), poor job performance and reduced productivity. Optimum daytime performance with minimal sleepiness in adolescents and young adults appears to require at least eight to nine hours of sleep at night with few interruptions. A majority of adolescents and adults, however, report habitual sleep durations of fewer than seven hours per night during the week and fewer than eight hours of sleep each night on weekends.
- The beneficial effects of healthy sleep habits and the adverse consequences of poor or insufficient sleep have not been well studied. Sleep is essential for survival, yet only in the last decade has scientifically credible, experimentally-based data from humans been gathered on dose-response relationships between chronic restriction of sleep by one to four hours a night and accumulating daytime sleepiness and cognitive impairments. Most individuals develop cognitive deficits from chronic sleep debt after only a few nights of reduced sleep quality or quantity, and new evidence suggests additional important health-related consequences from sleep debt related, for example, to common viral illnesses, diabetes, obesity, heart disease, and depression. Findings from a recent study of young adult men placed on a restricted sleep schedule of four hours each night for six consecutive nights showed altered metabolism of glucose with an insulin resistance pattern similar to that observed in elderly men. The implications from this study, if replicated, are that chronic sleep loss may contribute to obesity, diabetes, heart disease, and other age-related chronic disorders. As promising as these data are for providing solid scientific evidence of the health consequences of chronic insufficient sleep, most people report habitual nighttime sleep in the range of 6 hours. Data are needed to determine the extent to which habitual sleep durations of 6 to eight 8 hours are associated with increased disease risk in men and in women.
Sleep Duration and Quality: Relationship to Morbidity and Mortality:
- The relationship between sleep (quantity and quality) and estimates of morbidity and mortality remains controversial. Data from epidemiological studies suggest that a habitual short sleep duration (less than six hours sleep per night) or long sleep duration (more than nine hours sleep per night) is associated with increased mortality. A recent epidemiological report found that self-reported sleep duration averaging either less or more than seven hours of sleep daily was associated with higher mortality. It is not clear how sleep duration increases risk. Moreover, although such epidemiological studies have used very large convenience samples, they have relied on retrospective self-report, the least accurate index of sleep.
- There have been no epidemiological prospective studies examining the relationship between sleep and health outcomes (morbidity and mortality) that included estimates of sleep based on both subjective and objective measures. Past practices of adding questions about habitual sleep duration to large epidemiological studies designed to answer questions about, for example, the relationship between nutrition and risk for heart disease or between smoking and cancer risk are not sufficient. Although studies of sleep patterns and behavior would be prohibitively expensive and, require multiple sites with subjective and objective measures of sleep in a very large sample, important questions regarding the relationship between sleep duration and quality and morbidity and mortality can only be addressed through such large studies. Furthermore, recent studies have shown that sleep duration of at least eight hours is necessary for optimal performance and to prevent physiological daytime sleepiness and the accumulation of 'sleep debt'. Findings from these and other studies can only be reconciled with data suggesting that habitual sleep durations of eight hours is associated with higher mortality by a large comprehensive study of the effects of sleep on health and risk for disease.
Without knowledge of what needs sleep fulfills or what sleep patterns (duration and quality) best predict health (or morbidity and mortality) it is very difficult for sleep researchers and clinicians to answer questions such as:
What are normal sleep patterns?
How much sleep is needed by infants, children, teenagers, adults and the elderly for healthy functioning?
What is the minimum amount of sleep required for optimal functioning and for health?
How much of the patterning of sleep is genetic and how much is environmental?
How are patterns of growth and development from infancy to adolescence and from adolescence to adulthood negatively impacted by insufficient sleep?
What is the influence of prematurity on the development of sleep patterns? Do infants considered 'poor or problem' sleepers develop Insomnia as children and/or adults?
What sleep patterns in young and middle-aged adults predict good quality sleep in the elderly? Is the amount of sleep more important than the time of day when sleep occurs?
Do daytime naps make up for lost sleep at night?
What are the health consequences of long-term exposure to chronic sleep restriction such as those imposed by a typical work or school schedule of five or more days of sleep restriction followed by 2 days of partial recovery?
Does the duration of recovery sleep, or the timing of recovery sleep in the daily cycle, or both, determine the benefits of sleep for healthy functioning?
How much recovery sleep is required following exposure to chronic sleep restriction in order to restore physiological and neurobehavioral capability to baseline?
Does habituation/adaptation in the body's physiological systems develop to sleep loss or chronic sleep restriction and at what cost to one's health?
How important are environmental variables such as sleep surface and light) in affecting sleep quality and quantity?
- Epidemiological longitudinal studies to define normal sleep behavior (timing, duration, quality) and phenotypes using state-of-the art objective and subjective technologies. Such studies are needed across the life span and especially in vulnerable populations, and should focus on the transition from infancy to adolescence, adolescence to middle age, and middle age to advanced age. Prospective studies representing the diversity of human cultures in the US are needed to define normal sleep phenotypes across cultures and in different ethnic and socioeconomic groups.
- Epidemiological longitudinal studies to prospectively assess the relationships among sleep duration (short and long), sleep quality (good and poor), and health outcomes (morbidity and mortality). There is a need to determine the incidence and prevalence of 'sleep debt' in vulnerable populations including children, adolescents, young adults, shift workers, new parents. those exposed to prolonged work hours, those of low socioeconomic status. There is also a need to determine the functional and health-related consequences of sleep debt on increased disease risk such as related to depression, obesity, diabetes, and cardiopulmonary diseases.
- The genetic, environmental, and psychosocial factors that impact children's sleep and the relative contributions of each need to be identified.
- Identify how sleep disturbances in early childhood impact cognitive and social development, behavior and performance, as well as subsequent development of sleep disorders in adolescents and adults.
- Studies to identify and evaluate countermeasures and novel approaches to prevent sleep deprivation in children and adults. Countermeasures to improve the sleep of shift workers, especially daytime sleep after the night shift, are needed. Practical methods need to be developed to phase shift circadian rhythms to align with daytime sleep. Factors in need of study include: work and sleep schedule combinations, light exposure during night shifts (intensity, timing, duration, wavelength) and daylight exposure following completion of the night shift.
- The effects on sleep physiology and daytime functions of relevant environmental variables (in isolation and combination) need to be investigated, especially in vulnerable populations such as children, adolescents, the elderly, socioeconomically disadvantaged, and shift workers. Factors to study include: sleeping alone versus sleeping with one or more other persons in the bed (including children), sleep position, sleep surface, personal risk, and environmental stimuli such as noise, vibration, light, temperature, and humidity.
Reprinted from http://www.nhlbi.nih.gov
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