Age differences in sleep-wake behavior under natural conditions

The Emerging Importance of Sleep Regularity on Cardiovascular Health and Cognitive Impairment in Older Adults: A Review of the Literature

The regularity of sleep/wake patterns across multiple days is emerging as an important determinant of health. However, the association between sleep regularity and health outcomes in the aging population is not well understood. The current systematic review identified 22 publications that examined the relationship between sleep regularity and selected health outcomes: cardiovascular risk, cognitive impairment, and mortality. All studies were published after 2010, reflecting a growing research interest in daily sleep regularity. Low sleep regularity was consistently associated with higher cardiovascular risk and elevated risk of all-cause mortality. Results on cognitive impairment are mixed, with inconsistency likely attributed to small sample sizes and differences in sleep regularity assessment. Overall, regularity in sleep carries important information about health and should be included in future studies that collect daily sleep measures. Gaps in literature and methodological shortcomings are discussed.

Sleep variability and regularity as contributors to obesity and cardiometabolic health in adolescence

OBJECTIVE

Adolescence is a developmental stage of critical changes in sleep and its circadian timing when the contribution of abnormal sleep variability (amount) and sleep regularity (timing) to obesity and its associated adverse cardiometabolic health outcomes appears to increase. The aim of this study was to summarize findings from studies conducted in adolescents examining both sleep variability and regularity in relation to obesity and cardiometabolic health. Gaps in research and potential causal pathways that future studies should examine are highlighted.

RESULTS

Nightly deviations in sleep duration and sleep midpoint appear to contribute to the development of obesity and associated adverse cardiometabolic outcomes in youth. Studies show that increased sleep variability and irregularity are associated with obesity, decreased physical activity, dysregulated eating and inadequate diet, metabolic dysfunction, impaired cardiac autonomic balance, and elevated blood pressure in adolescents.

CONCLUSIONS

A stable circadian timing of sleep is essential to the overall physical well-being of youth. Emerging evidence supports that sleep variability and circadian misalignment, including sleep irregularity, contribute to adverse obesity-related health outcomes early on in adolescence. Future studies should focus on the underlying behavioral and biological mechanisms in the causal pathway between day-to-day deviations in the amount and timing of sleep and obesity.

Sleep duration irregularity is associated with elevated blood pressure in young adults

Sleep irregularity (i.e., highly variable sleep patterns) is an emerging risk factor for cardiometabolic disease. Though irregular sleep patterns are common among young adults, the cardiometabolic health (CMH) repercussions of sleep irregularity in this population are unclear. We examined associations between sleep duration and irregularity with measures of CMH in 44 (24 M/20 F, 23 ± 5y, BMI 26 ± 4 kg/m², blood pressure (BP): 125/71 ± 14/9 mmHg) young adults. Participants wore actigraphy monitors for seven-days and sleep duration irregularity was operationalized as the standard deviation of nightly sleep duration (sleep SD). CMH variables of interest included brachial and aortic BP, arterial stiffness (cf-PWV), augmentation index (AIx75), and fasting blood glucose and lipids. Associations between sleep duration and sleep SD with CMH variables were assessed via correlations adjusted for sex and BMI. Sleep duration generally was not associated with CMH indices. However, sleep SD was associated with brachial systolic (r = 0.433, p = .027) and diastolic BP (r = 0.415, p = .035). Similarly, sleep duration SD was associated with aortic systolic BP (r = 0.447, p = .022). Our findings show that sleep irregularity, but not duration, is associated with higher brachial and central BP in young adults.Abbreviations: AIx75: augmentation index at a heart rate of 75 beats per minute; BP: blood pressure; CMH: cardiometabolic health; cf-PWV: carotid-femoral pulse wave velocity; DXA: dual x-ray absorptiometry; mg/dl: milligrams per deciliter; PWA: pulse wave analysis; PWV: pulse wave velocity; sleep duration SD: standard deviation of nightly sleep duration.

Cardiovascular risks and sociodemographic correlates of multidimensional sleep phenotypes in two samples of US adults

Study Objectives

Sleep is a modifiable risk factor for cardiovascular conditions. Holistic examination of within-person, multidimensional sleep patterns may offer more detailed information about the sleep-cardiovascular condition link, including who is more vulnerable to both. This study aimed to identify common sleep phenotypes in adulthood, establish the validity of the phenotypes in relation to cardiovascular conditions, and explore sociodemographic and background characteristics of the phenotypes.

Methods

Across two independent samples of adults (N ₁ = 4600; N ₂ = 2598) from the Midlife in the United States Study, latent class analysis (LCA) extracted sleep phenotypes using five key self-reported sleep dimensions. Log-binomial regression was used to determine whether sleep phenotypes differentially predicted cardiovascular conditions, adjusting for known risk factors. LCA with covariates was used to compare sociodemographic characteristics of the identified sleep phenotypes.

Results

Four sleep phenotypes were identified consistently across the two samples: good sleepers, nappers, dissatisfied/inefficient sleepers, and irregular sleepers. Compared to good sleepers (reference), dissatisfied/inefficient sleepers exhibited a higher risk of cardiovascular conditions in both samples (RR _Sample1: 29%, RR _Sample2: 53%) and consisted of relatively more racial/ethnic minorities. Nappers exhibited a higher risk of cardiovascular conditions in one sample (RR _Sample1: 38%) and consisted of more women and older adults. Irregular sleepers exhibited no significantly different cardiovascular risk and were relatively younger.

Conclusions

Common sleep phenotypes in adulthood exhibit differential risks for cardiovascular conditions. Cooccurring sleep dissatisfaction and inefficiency, in particular, may relate to increased risk of cardiovascular conditions. Certain sociodemographic groups (racial minorities, women, older adults) disproportionately fit within high-risk sleep phenotypes.

The Association Between Dry Eye and Sleep Disorders: The Evidence and Possible Mechanisms

Dry eye is a disease that severely affects patients' quality of life, increasing the global burden on public health and finance. There is growing evidence that a poor lifestyle is a significant risk factor for dry eye. Along with the development of society, sleep, as a way of life, is also constantly changing. The main manifestations of sleep disorders are reduced sleep time, circadian rhythm disturbances, and sleep breathing disturbances. Sleep disorders and their secondary systemic diseases have attracted wide attention in recent years. This review mainly explored the correlation between sleep disorders and dry eye, and found that sleep-related problems and other factors potentially leading from sleep disorders could be critical factors for dry eye. These results suggest that ophthalmologists should pay attention to the sleep health problems in patients with dry eye, and we hope that this paper can provide help for future research in this field.

Biological Rhythm and Chronotype: New Perspectives in Health

The circadian rhythm plays a fundamental role in regulating biological functions, including sleep–wake preference, body temperature, hormonal secretion, food intake, and cognitive and physical performance. Alterations in circadian rhythm can lead to chronic disease and impaired sleep. The circadian rhythmicity in human beings is represented by a complex phenotype. Indeed, over a 24-h period, a person’s preferred time to be more active or to sleep can be expressed in the concept of morningness–eveningness. Three chronotypes are distinguished: Morning, Neither, and Evening-types. Interindividual differences in chronotypes need to be considered to reduce the negative effects of circadian disruptions on health. In the present review, we examine the bi-directional influences of the rest–activity circadian rhythm and sleep–wake cycle in chronic pathologies and disorders. We analyze the concept and the main characteristics of the three chronotypes.

Impact of an Individually Tailored Light Mask on Sleep Parameters in Older Adults With Advanced Phase Sleep Disorder

Objective: This study investigated whether light delivered through the eyelids of sleeping persons might create phase delay in older adults who are adversely affected by advanced sleep phase disorder. Participants: Thirty-two cognitively intact, community-dwelling participants aged ≥ 50 years (20 females, 12 males) with Pittsburgh Sleep Quality Index scores ≥ 5 (poor sleep) completed the study. Methods: This within-subjects, randomized, two-treatment crossover design study exposed participants to an active "blue" (λ_max  =  480 nm) lighting intervention or a placebo "red" (λ_max = 640 nm) control through closed eyelids during sleep for 8 weeks. Conditions were administered 1 hr after bedtime using custom-built light masks delivering a train of 2-s duration light pulses presented every 30 s for ≤ 2 hr (approximately 240 pulses/night). Dependent variables were subjective measures of sleep and depression (questionnaires) and objective measures of sleep (wrist actigraphy), analyzed using linear mixed models with treatment, period, and carryover as fixed effects. Results: The actigraphy analysis found no effect of the intervention or the control condition on sleep start time, total sleep time, number of sleep bouts, or sleep efficiency, either compared to baseline or to one another. Subjective responses of study participants, however, indicated statistically significant (p < 0.05) improvement in seven of eight reported measures of sleep quality with both the intervention and the control condition, but no difference between the two conditions. Conclusions: The participants reported improvement in sleep quality, but the intervention did not confer additional advantages after adjusting for period and carryover effects.

Reduced impact of alcohol use on next-day tiredness in older relative to younger adults: A role for sleep duration

Recent work has suggested that older adults may be less susceptible to the next-day effects of alcohol relative to younger adults. The effects of alcohol in younger adults may be mediated by sleep duration, but due to age differences in the contexts of alcohol use, this mediation process may not generalize to older adults. The present study examined age-group (younger vs. older adults) differences in how alcohol use influenced next-day tiredness during daily life. Reports of alcohol use, sleep duration, and next-day tiredness obtained on ∼101 days from 91 younger adults (ages 20-31 years) and 75 older adults (ages 65-80 years) were modeled using a multilevel, moderated mediation framework. Findings indicated that (a) greater-than-usual alcohol use was associated with greater-than-usual tiredness in younger adults only, (b) greater-than-usual alcohol use was associated with shorter-than-usual sleep duration in younger adults only, and (c) shorter-than-usual sleep duration was associated with greater tiredness in both younger and older adults. For the prototypical younger adult, a significant portion (43%) of the association between alcohol use and next-day tiredness could be explained assuming mediation through sleep duration, whereas there was no evidence of mediation for the prototypical older adult. Findings of age differences in the mediation process underlying associations among alcohol use, sleep, and tiredness provide insight into the mechanisms driving recent observations of reduced next-day effects of alcohol in older relative to younger adults. (PsycINFO Database Record

Sleep disorder or simple sleep ontogeny? Tendency for morningness is associated with worse sleep quality in the elderly

The objective of this study was to evaluate the alterations in sleep and circadian parameters during the aging process. The study sample comprises volunteers older than 18 up to 90 years of age that answered the Pittsburgh Sleep Quality Index (PSQI) and the Horne and Östberg circadian preference questionnaire. We observed that the shift to morningness with increasing age is associated with a significant worsening in sleep quality. We discuss that this sleep profile characterized by morningness and worse sleep quality observed in elderly, when compared to younger people, reflects not necessarily a pathological state, but an expected profile for this age group.

Longitudinal Stability of Genetic and Environmental Influences on the Association between Diurnal Preference and Sleep Quality in Young Adult Twins and Siblings

Overlapping genetic influences have been implicated in diurnal preference and subjective sleep quality. Our overall aim was to examine overlapping concurrent and longitudinal genetic and environmental effects on diurnal preference and sleep quality over ~5 years. Behavioral genetic analyses were performed on data from the longitudinal British G1219 study of young adult twins and nontwin siblings. A total of 1556 twins and siblings provided data on diurnal preference (Morningness-Eveningness Questionnaire) and sleep quality (Pittsburgh Sleep Quality Index) at time 1 (mean age = 20.30 years, SD = 1.76; 62% female), and 862 participated at time 2 (mean age = 25.30 years, SD = 1.81; 66% female). Preference for eveningness was associated with poorer sleep quality at both time points ( r = 0.25 [95% confidence intervals {CIs} = 0.20-0.30] and r = 0.21 [CI = 0.15-0.28]). There was substantial overlap in the genetic influences on diurnal preference and sleep quality individually, across time (genetic correlations [rAs]: 0.64 [95% CI = 0.59-0.67] and 0.48 [95% CI = 0.42-.053]). There were moderate genetic correlations between diurnal preference and sleep quality concurrently and longitudinally (rAs = 0.29-0.60). Nonshared environmental overlap was substantially smaller for all cross-phenotype associations (nonshared environmental correlations (rEs) = -0.02 to 0.08). All concurrent and longitudinal associations within and between phenotypes were largely accounted for by genetic factors (explaining between 60% and 100% of the associations). All shared environmental effects were nonsignificant. Nonshared environmental influences played a smaller role on the associations between phenotypes (explaining between -0.06% and 40% of the associations). These results suggest that to some extent, similar genes contribute to the stability of diurnal preference and sleep quality throughout young adulthood but also that different genes play a part over this relatively short time frame. While there was evidence of genetic overlap between phenotypes concurrently and longitudinally, the possible emergence of new genetic factors (or decline of previously associated factors) suggests that molecular genetic studies focusing on young adults should consider more tightly specified age groups, given that genetic effects may be time specific.

Beyond the mean: A systematic review on the correlates of daily intraindividual variability of sleep/wake patterns

Features of an individual's sleep/wake patterns across multiple days are governed by two dimensions, the mean and the intraindividual variability (IIV). The existing literature focuses on the means, while the nature and correlates of sleep/wake IIV are not well understood. A systematic search of records in five major databases from inception to November 2014 identified 53 peer-reviewed empirical publications that examined correlates of sleep/wake IIV in adults. Overall, this literature appeared unsystematic and post hoc, with under-developed theoretical frameworks and inconsistent methodologies. Correlates most consistently associated with greater IIV in one or more aspects of sleep/wake patterns were: younger age, non-White race/ethnicity, living alone, physical health conditions, higher body mass index, weight gain, bipolar and unipolar depression symptomatology, stress, and evening chronotype; symptoms of insomnia and poor sleep were associated with higher sleep/wake IIV, which was reduced following sleep interventions. The effects of experimentally reduced sleep/wake IIV on daytime functioning were inconclusive. In extending current understanding of sleep/wake patterns beyond the mean values, IIV should be incorporated as an additional dimension when sleep is examined across multiple days. Theoretical and methodological shortcomings in the existing literature, and opportunities for future research are discussed.

Variability in self-reported normal sleep across the adult age span

OBJECTIVES

Illustrate the importance of examining within- and between-person differences in sleep across the adult age span.

METHOD

Two weeks of sleep diary data were analyzed for 592 normal sleepers ranging in age from 20 to 96 years. Variability in total sleep time (TST), number of nighttime awakenings (NWAK), sleep-onset latency (SOL), and wake-time after sleep onset (WASO) were examined overall and by age, sex, and race utilizing multilevel models and multiple regression.

RESULTS

Night-to-night differences in sleep within the same individual generally exceeded differences between individuals for TST, SOL, and WASO. The amount of intraindividual variability in TST and NWAK decreased with older age. Further, the degree of reduction in variability in TST associated with age depended on sex and race, with young black females showing the greatest variability. In general, females tended to have more intraindividual variability in SOL and NWAK than males, while race differences were complicated by high variability between blacks.

DISCUSSION

To truly assess and understand individual differences in the sleep of older adults, future research needs to take into account night-to-night variability (including what makes sleep vary from one night to the next), in addition to average sleep.

Sleep quality and diurnal preference in a sample of young adults: associations with 5HTTLPR, PER3, and CLOCK 3111

Research investigating associations between specific genes and individual differences with regards to the quality and timing of sleep has primarily focussed on serotonin-related and clock genes. However, there are only a few studies of this type and most of those to date have not considered the possibility of gene-environment interaction. Here, we describe associations between sleep quality and diurnal preference and three functional polymorphisms: 5HTTLPR, PERIOD3, and CLOCK 3111. Furthermore, we assessed whether associations between genotypes and sleep phenotypes were moderated by negative life events-a test of gene-environment interaction. DNA from buccal swabs was collected from 947 individuals [mean age = 20.3 years (SD = 1.77), age range = 18-27 years; 61.8% female] and genotyped for the three polymorphisms. Participants completed the Pittsburgh Sleep Quality Index and the Morningness-Eveningness Questionnaire. There was a significant main effect of 5HTTLPR on sleep quality, indicating that "long-long" homozygotes experienced significantly poorer sleep quality (mean = 6.35, SD = 3.36) than carriers of at least one "short" allele (mean = 5.67, SD = 2.96; β = -0.34, P = 0.005). There were no main effects of 5HTTLPR on diurnal preference; no main effects of PERIOD3 or CLOCK on sleep quality or diurnal preference; and no significant interactions with negative life events. The main effect of the "long" 5HTTLPR allele contradicts previous research, suggesting that perhaps the effects of this gene are heterogeneous in different populations. Failure to replicate previous research in relation to PERIOD3 and CLOCK concurs with previous research suggesting that the effects of these genes are small and may be related to population composition.

[Relations between sleep patterns, perceived health and socioeconomic variables in a sample of community resident elders: PENSA Study]

Changes in sleep pattern constitute a common complaint among elders. Age-related changes can be observed in different levels of the architecture and quality of sleep. The objective of the present study was to observe the relation between age-related sleep changes, insomnia symptoms, apnea symptoms, parasomnia symptoms with socio-economic variables and perceived health status. Sleep pattern was evaluated using the Mini-Sleep Questionnaire. Results indicated that women and individuals with lower education reported higher frequency of insomnia symptoms, the elder-elder group. On the other hand, individuals with higher perceived health status presented lower frequency of apnea symptoms, and higher perceived health status was associated to better sleep pattern overall. It can be concluded that sleep pattern is associated to socio-economic variables such as age, gender, education and perceived health.

Genetic and environmental influences on different components of the Pittsburgh Sleep Quality Index and their overlap

STUDY OBJECTIVES

To examine the extent to which genetic and environmental factors influence components of sleep quality; the degree to which these components co-occur; and genetic and environmental influences on this co-occurrence.

DESIGN

Twin study.

SETTING

Population based twin registry across the U.K.

PATIENTS OR PARTICIPANTS

Four hundred twenty monozygotic twins, 773 dizygotic twins, and 363 siblings (mode age = 20 years; range 18 to 27 years).

INTERVENTIONS

N/A.

MEASUREMENTS AND RESULTS

The Pittsburgh Sleep Quality Index (PSQI) assessed 7 components of sleep quality which overlap to varying degrees. Genetic influence on individual components ranged from 0% to 47%. The remaining source of variance was non-shared environment, except for "sleep duration", for which shared environmental influences were important. Phenotypic correlations between components ranged from 0.22 to 0.61. Bivariate analyses indicated substantial overlap between genes influencing phenotypes (10 of 15 correlations were > or = 0.69); and in general, genetic influence accounted for roughly half the association (> 40% in 9 of 15 correlations). Non-shared environmental influences were in general less correlated across variables (11 of 15 were < 0.4), but owing to their greater influence on each variable, still accounted for roughly half of each association (> or = 40% in 12 of 15 correlations).

CONCLUSIONS

Genetic and non-shared environmental factors are most important in explaining individual differences with regards to different components of sleep quality, although shared environment may influence sleep duration. The pattern of overlap in the genetic and environmental influences accounting for the associations between components of sleep quality is consistent with that seen in other areas of developmental psychopathology of general genes and specific non-shared environmental influences.

Morningness-eveningness comparison in adolescents from different countries around the world

Very few studies have focused on differences in chronotype around the globe. In this study, morningness-eveningness was measured using the Composite Scale of Morningness and the midpoint of sleep on free days (corrected for sleep debt, MSF(sc)) in adolescents of sixteen German schools abroad, dispersed all over the world, and for comparison pupils attending German schools. There was no influence of duration of residence in the respective locality, and sex differences were weak. Age correlated negatively with morningness. A significant influence of climate zone (temperate, subtropics, tropics) was found, with adolescents in the subtropics being the latest chronotypes, and an interaction of agexclimate zone was identified. Significant associations between chronotype and longitude and latitude were identified within the time zone of central Europe. Adolescents were found to be more morning oriented both toward the East and North. The results indicate that climate, longitude, and latitude contribute to chronotype.

Actigraphy in older adults: comparison of means and variability of three different aggregates of measurement

Actigraphy has emerged as a valuable method for measuring natural sleep patterns; however, it is unclear how many consecutive nights should be measured and what sleep parameter values are typical of older adults. This study examined sleep in relatively healthy community-based older adults for 14 consecutive days. The findings present comparison values for Actiwatch-L actigraphy in older adults, and can be used to determine an appropriate length of measurement. When using means of the aggregate period, a 3-day aggregate is comparable to the values obtained over 7- or 14-day aggregates. However, if variability of the sleep parameter is of interest, a minimum of a 7-day aggregate is needed, and 14-day aggregates are better when measuring sleep onset latency.

Insomnia

The pathogenesis of insomnia in an individual usually is multifaceted. Effective treatments require a thorough evaluation to determine the factors that need to be addressed. Pharmacologic treatments generally are safe and effective for short-term use. Long-term hypnotic use remains controversial because of the potential risk of tolerance and dependency. Various cognitive and behavioral treatments for insomnia have been shown to be effective in the management of insomnia. For long-term follow-up, multicomponent cognitive behavioral therapy, alone or in combination with hypnotic use, has been shown to be superior to hypnotic use alone.

Self-Reported Napping and Nocturnal Sleep in Taiwanese Elderly Insomniacs

Abstract The aim of this study was to investigate the relationships between self-reported nocturnal sleep quality and napping patterns in elderly persons with insomnia and to compare the nocturnal sleep quality between napping and non-napping groups. Convenience sampling was used to recruit 60 community-dwelling elderly residents of Taichung City, Taiwan (age range 60-83 years, mean 67.1 years) who reported insomnia. All participants scored greater than 5 on the Pittsburgh Sleep Quality Index (PSQI) questionnaire. Napping prevalence, frequency, and duration were assessed by participant interview. Self-reported sleep quality, sleep latency, sleep duration, sleep efficiency, sleep disturbance, use of sleep medication, and daytime dysfunction were measured with the PSQI. Sixty-four percentage of participants (n = 38) reported napping. There were no age, gender, and ethnicity differences on napping patterns. Global sleep quality, sleep efficiency, and sleep disturbance were significantly associated with prevalence of napping (r = 0.24-0.26, p < 0.05). A significant correlation was also found between global sleep quality and nap duration (r = 0.31, p < 0.05). Elders in the napping group reported better global sleep quality (t = 2.2, p < 0.05) and sleep efficiency (t = 2.1, p < 0.05) than those in the non-napping group. The findings suggest that there is no need for health care providers to restrict elderly insomniacs' daytime napping.

Music improves sleep quality in older adults

AIM

The aim of this paper is to report an investigation of the effects of soft music on sleep quality in older community-dwelling men and women in Taiwan.

BACKGROUND

Sleep is a complex rhythmic state that may be affected by the ageing process. Few studies have focused on the effects of music, a non-pharmacological method of improving the quality of sleep in older adults.

METHOD

A randomized controlled trial was used with a two-group repeated measures design. Sixty people aged 60-83 years with difficulty in sleeping were recruited through community leaders and screened using the Pittsburgh Sleep Quality Index (PSQI) and Epworth Sleepiness Scale. Those reporting depression, cognitive impairment, medical or environmental problems that might interfere with sleep; and those who used sleeping medications, meditation, or caffeine at bedtime were excluded. Participants listened to their choice among six 45-minute sedative music tapes at bedtime for 3 weeks. There were five types of Western and one of Chinese music. Sleep quality was measured with the PSQI before the study and at three weekly post-tests. Groups were comparable on demographic variables, anxiety, depressive symptoms, physical activity, bedtime routine, herbal tea use, napping, pain, and pretest overall sleep quality.

RESULTS

Music resulted in significantly better sleep quality in the experimental group, as well as significantly better components of sleep quality: better perceived sleep quality, longer sleep duration, greater sleep efficiency, shorter sleep latency, less sleep disturbance and less daytime dysfunction (P = 0.04-0.001). Sleep improved weekly, indicating a cumulative dose effect.

CONCLUSION

The findings provide evidence for the use of soothing music as an empirically-based intervention for sleep in older people.

Nutritional status, growth and sleep habits among Senegalese adolescent girls

OBJECTIVE

To study the relation between sleep habits, nutritional status, growth and maturation in a group of African adolescent girls. The main hypothesis to be tested was that sleep length could be an effective way to spare energy, and thus malnourished girls sleep longer than normal girls.

DESIGN

Three repeated yearly surveys (1997-1999) on a subsample of girls drawn from a larger study cohort on growth at adolescence.

SETTING

The Niakhar district in the central part of Senegal.

SUBJECT

In total, 40 girls were initially drawn. Missing girls were replaced at each round by girls having the same characteristics and belonging to the same cohort.

INTERVENTION

At each round, data on pubertal development (breast stages and occurrence of menarche), growth and nutritional status were collected. Adolescents wore an accelerometer for three or four consecutive nights and days at each round.

RESULTS

At the beginning of the survey, girls were 13.3+/-0.5 y old. They were under international reference values in weight and height. Their mean sleep duration was 8.5+/-0.9 h. Their puberty status did not influence their sleep habits; however, they slept more in March than in June, which was related to the seasonal change in daylight. There was a significant relation between body mass index and sleep habits: thinner girls slept a longer time and more quietly than the more corpulent girls.

CONCLUSION

The nutritional status of these girls influenced their sleep habits: this may have been either a direct causal relation or a consequence of a protective attitude on the part of the mothers towards the frailer girls.

The influence of heredity on self-reported sleep patterns in free-living humans

The heritability of self-reported sleep patterns was investigated with 86 identical and 78 fraternal same-sex and 51 fraternal mixed-gender adult twin pairs who were paid to maintain 7-day diaries. Linear structural modeling was applied to investigate the nature and degree of genetic and environmental influences and revealed significant genetic influences on the time that individuals went to sleep and woke up, how often the individual woke up during the night, the duration of sleep and wakefulness, and how alert the individual felt upon waking and over the day, accounting for 21% to 41% of the variance. These influences of heredity were present for sleep-wake behavior over the entire week and also when the sleep-wake pattern was analyzed separately for weekdays and weekends. Further, it was demonstrated that there were multiple independent influences of heredity on sleep-wake behavior. The results suggest that sleep-wake patterns are not learned but result in part from multiple heritable influences.

Age-associated difference in circadian sleep-wake and rest-activity rhythms

Using actigraphic monitoring of wrist activity, we investigated the sleep and rest-activity patterns of 65 young, middle-aged, old and the oldest subjects in their natural environmental conditions. To assess the effects of age and gender on sleep and circadian rhythms in activity, multivariate analyses were performed. Age significantly affected circadian sleep and rest-activity rhythms. In the old and oldest groups, the actigraphic estimates of "actual sleep time" and "sleep efficiency" decreased significantly. The estimates of "sleep latency," the number of "nighttime awakening," sleep fragmentation and daytime naps significantly increased in the old and oldest groups. Concerning the circadian patterning of rest and activity, the interdaily stability (IS) was similar in the four age groups, while the old and oldest subjects showed significant increases in intradaily variability (IV) and nighttime activity and a decrease in amplitude (AMP). The present study demonstrated weakened and fragmented circadian sleep and rest-activity rhythms during aging. However, no gender-related difference was found.

The role of actigraphy in sleep medicine

During the last decade actigraphy (activity-based monitoring) has become an essential tool in sleep research and sleep medicine. The validity, reliability and limitations of actigraphy for documenting sleep-wake patterns have been addressed. Normative data on sleep-wake patterns across development have been collected. Multiple studies have documented the adequacy of actigraphy to distinguish between clinical groups and to identify certain sleep-wake disorders. Actigraphy has also been shown to be effective in documenting the effects of various behavioral and medical interventions on sleep-wake patterns. Actigraphy is less useful for documenting sleep-wake in individuals who have long motionless periods of wakefulness (e.g. insomnia patients) or who have disorders that involve altered motility patterns (e.g. sleep apnea). Potential users should be aware of a number of pitfalls of actigraphy: (1) validity has not been established for all scoring algorithms or devices, or for all clinical groups; (2) actigraphy is not sufficient for diagnosis of sleep disorders in individuals with motor disorders or high motility during sleep; (3) the use of computer scoring algorithms without controlling for potential artifacts can lead to inaccurate and misleading results.