Clinical ReviewNapping: A public health issue. From epidemiological to laboratory studies
Introduction
Over the last few decades, epidemiological studies have progressively reported that sleep loss is a clear independent risk factor associated with obesity, type 2 diabetes, cardiovascular disease (CVD), and accidents [1], [2], [3], [4]. In laboratory research, it has also been clearly shown that sleep restriction is associated with immune, endocrine, and vascular dysfunction [5], [6], ∗[7], [8], [9]. It is now well established that, on average, 10–35% of adults suffer from sleep loss during weekdays, sleeping less than 6 h per 24 h [10], [11], [12], [13]. One major determinant of sleep loss is work: in particular, 20% of the working population work at night or in shifts, and they sleep an average of one hour less than day workers [11], [14]. Moreover, the time taken to travel from home to work and back has increased in most cities, directly impacting total sleep time (TST) [15]. Another determinant negatively impacting sleep is the use of mobile electronic devices (phones, computers) and the increasing amounts of leisure time spent on the internet. This activity affects many young adults, drastically reducing TST [16].
To counteract the negative short- and long-term consequences of sleep debt, napping has often been proposed as a potential and powerful “public health tool”. In his book “The promise of sleep”, William C Dement wrote, “My fellow sleep specialists and I are campaigning to rehabilitate napping and demonstrate that taking naps is an excellent and respectable strategy for sleep management. Naps can make you smarter, faster, and safer than you would be without them. They should be widely recognized as a powerful tool in battling fatigue, and the person who chooses to nap should be regarded as heroic” [17]. However, napping is not at all a homogeneous habit around the world, and this heterogeneity has to be taken into consideration from the public health point of view. It is indeed not the same to have a nap once a week on weekends and to nap on a daily basis due to shift work. While the short-term effects of napping on attention, concentration, and reaction time may take effect with a single nap, the mid- or long-term benefits of napping may only be considered in regular nappers (i.e., napping several times a week). Thus, different types of naps, depending on the length, the circadian position, and the sleep architecture, have to be defined [18]. Napping habits also change during an individual's lifespan, from a progressive decrease in nap length and nap episodes during early childhood to the midday napping habits practiced during adulthood [19]. Napping is also a culturally driven behaviour: afternoon/midday sleep or a siesta is a common habit in Mediterranean culture, mainly for climatic reasons, and in China, Taiwan, and Japan as a countermeasure to excessive working hours. In China, the right to nap at work has been written into the constitution since 1948 in order to ensure better working conditions and productivity. To embrace the diversity of napping while maintaining perspective on this rather universal and homogenous biological phenomenon, we will mostly consider, in this review, napping as a voluntary episode of sleep ranging from several minutes to nearly 90 min of sleep. Importantly, this review will not address naps that are taken as a result of sleep disorders, such as sleep apnoea.
At the beginning of the epidemiology section, we will present napping data during childhood as well as in the elderly, but we will focus mainly on young and middle-aged adults, as the need for efficient countermeasures to sleep-debt seems greatest in this latter population. Indeed, young people are more sensitive to sleep loss and are expected to have greater homeostatic pressure, so napping is a particularly effective countermeasure in younger adults [20]. We will also not forget napping issues related to professional life (night and shift work schedules).
The present review may be criticised for focusing predominantly on the beneficial effects of napping, but we also briefly describe how napping may have deleterious effects. One frequently claimed deleterious effect is sleep inertia, which is generally defined as a sensation of disorientation and a transient reduction in cognitive performance following long naps exceeding 30 min [21]. Usually, the longer the nap, the higher the percentage of slow-wave sleep (SWS) and the greater the occurrence of sleep inertia. These effects may be cancelled out by caffeine when taken right after the nap [22], [23], [24].
Another deleterious effect is associated with long naps during the afternoon or evening. These naps have been described to disturb sleep latency and both the quality and quantity of the subsequent night-time sleep. Accordingly, the usual recommendation is to restrict daytime napping for patients with insomnia (especially sleep onset insomnia) or delayed sleep phase syndrome. Studies in healthy subjects, in contrast, have led to contradictory conclusions. Objective recordings of sleep, such as at-home actigraphy, have shown that napping an average of 28.0 min/d between 12:00 h and 18:00 h in young subjects (mean age 23.8 ± 3.8 y old) does not influence nocturnal sleep [25]. In the elderly, however, napping from 30 min to 90 min during the afternoon and early evening seems more controversial in terms of its effects on night-time sleep. Yoon et al. reported in this same ecological study that “young elderly” nappers (mean age 66.2 ± 4.9 y old napping for an average of 30.4 min/d) had shorter time in bed and an earlier wake-up time but no significant differences in TST or sleep efficiency. In older nappers (mean age 78.6 y old, napping for 57 ± 18 min at the beginning of the afternoon), Monk et al. [26] measured lower sleep efficiency during night-time sleep using actigraphy and polysomnography (PSG) but found no change in SWS or wake time after sleep onset (WASO) and better objective evening vigilance performance. Finally, in the elderly, as discussed below, a long and potentially unplanned nap during daytime could be a contributing factor to increased cardiovascular risk (CVR).
Therefore, we will consider another important issue on how to objectively assess napping: not only the duration but also the quality of sleep in terms of non-REM (NREM) or rapid eye movement (REM) sleep. Several recommendations have been made, mostly by the American Academy of Sleep Medicine, regarding the use of PSG and actigraphy [27]. However, the lack of consensus on some methodological and theoretical points may impact the objective evaluation of naps: i) Do studies have to recruit habitual or non-habitual nappers? ii) Do scorers have to use specific classifications when scoring naps (sleep latency, first stage 1, SOREMs, awakenings)? iii) Do studies have to test naps at different times of the day or at midday? Interestingly, Kanady et al. tried to validate actigraphy versus PSG in the assessment of daytime naps and no-nap rest periods [28]. They tested the sensitivity levels (high, medium, low) of three actigraphy monitors (Actiwatch-64-Respironics) and compared sleep latency, TST, WASO, and sleep efficiency in 30 and 27 subjects in the nap and no-nap groups, respectively. They found that actigraphy was able to predict TST, sleep latency, and sleep efficiency during a nap, but with optimal levels depending on the variables of interest. A medium level of actigraphy monitor sensitivity showed significant correlations with TST and sleep efficiency, while high levels predicted sleep latency. Low-sensitivity levels were best for determining WASO. Actigraphy also accurately distinguished nap from rest in no-nap subjects, indicating that it is a reliable tool for the assessment of napping.
Finally, in the first section dealing with the epidemiology of napping, we will review the following: 1) napping across the lifespan; 2) sleep debt and napping habits; 3) napping in practical settings (sleep-related accidents, work and school); and 4) napping and cardiovascular risk. The second section will focus on the cognitive and physiological effects of napping, and we will review napping effects on 1) alertness; 2) memory; 3) stress and cardiovascular systems; 4) immune functions; and 5) pain sensitivity.
Section snippets
Epidemiology of napping
In the present section, we will fist describe napping across the lifespan and then discuss the circumstances in which napping could be a potential public health tool counteracting the consequences of sleep debt in real-life and clinical settings (Fig. 1).
Cognitive and physiological effects of napping
Here, we will review the beneficial physiological effects that have been reported to be associated with napping, with most of the data coming from healthy young adults or middle-aged adults in laboratory studies (Fig. 1).
Conclusions
In the present review, we have attempted to discuss the circumstances in which napping could be a potential public health tool as a measure to counteract sleep debt consequences in real-life situations (driving, work, and school) and for cardiovascular risk outcomes. Laboratory-based studies detailed the beneficial effects of napping on several systems (cognitive, stress, immune or pain functions). Furthermore, pilot nap-based laboratory studies have reported that a midday nap of 20–30 min
Conflicts of interest
Damien Leger has received funding or has been the main investigator in studies sponsored by Sanofi aventis, Merck, Vanda, Actelion, Bioprojet, Philips, Resmed, and Vitalaire in the last 5 years. Marie-Francoise VECCHIERINI has received funding or has been an investigator in studies sponsored by Sanofi aventis, Merck, Vanda, Bioproject, and Resmed in the last 5 years.
The other authors do not declare any conflict of interest.
Acknowledgements
We would like to thank Karen Pickett for English-language editing and Caroline Gauriau for the figure layout in the manuscript.
References∗ (195)
- et al.
Short and long sleep are positively associated with obesity, diabetes, hypertension, and cardiovascular disease among adults in the United States
Soc Sci Med
(2010) - et al.
The metabolic consequences of sleep deprivation
Sleep Med Rev
(2007) - et al.
Cardiovascular, inflammatory, and metabolic consequences of sleep deprivation
Prog Cardiovasc Dis
(2009) - et al.
Benefits of napping and an extended duration of recovery sleep on alertness and immune cells after acute sleep restriction
Brain Behav Immun
(2011) - et al.
Immune, inflammatory and cardiovascular consequences of sleep restriction and recovery
Sleep Med Rev
(2012) - et al.
Vascular response to 1 week of sleep restriction in healthy subjects. A metabolic response?
Int J Cardiol
(2015) - et al.
Short sleep in young adults: insomnia or sleep debt? Prevalence and clinical description of short sleep in a representative sample of 1004 young adults from France
Sleep Med
(2011) - et al.
Sleep inertia
Sleep Med Rev
(2000) - et al.
Relationship between napping pattern and nocturnal sleep among Japanese nursery school children
Sleep Med
(2012) - et al.
Napping in English preschool children and the association with parents’attitudes
Sleep Med
(2013)
Cross-cultural differences in the sleep of preschool children
Sleep Med
Sleep and sleep habits from childhood to young adulthood over a 10-year period
J Psychosom Res
Ultrashort sleep-waking schedule: gates and forbidden zones for sleep
Electroencephal Clin Neurophysiol
Performance and sleepiness during a 24 h wake in constant conditions are affected by diet
Biol Psychol
The role of prescribed napping in sleep medicine
Sleep Med Rev
Ethnic-specific associations of sleep duration and daytime napping with prevalent type 2 diabetes in postmenopausal women
Sleep Med
Naps, cognition and performance
Sleep Med Rev
Sleepiness, attention and risk of accidents in powered two-wheelers
Sleep Med Rev
The factors associated with preferences for napping and drinking coffee as countermeasures for sleepiness at the wheel among Japanese drivers
Sleep Med
Underexposure to light at work and its association to insomnia and sleepiness: a cross-sectional study of 13,296 workers of one transportation company
J Psychosom Res
Did a brief nap break have positive benefits on information processing among nurses working on the first 8-h night shift?
Appl Ergon
Night shift work, sleep duration, daytime napping, and breast cancer
Sleep Med
Does a siesta protect from coronary heart disease?
Lancet
Daytime napping and mortality from all causes, cardiovascular disease, and cancer: a meta-analysis of prospective cohort studies
Sleep Med
Longer habitual napping is associated with a higher risk for impaired fasting plasma glucose and diabetes mellitus in older adults: results from the Dongfeng-Tongji cohort of retired workers
Sleep Med
Quantity and quality of sleep and incidence of type 2 diabetes: a systematic review and meta-analysis
Diabetes Care
Short sleep duration and increased risk of hypertension: a primary care medicine investigation
J Hypertens
Complaints of poor sleep and risk of traffic accidents: a population-based case-control study
PLoS One
Factors associated with sleep duration in Korean adults: results of a 2008 community health survey in Gwangju metropolitan city, Korea
J Korean Med Sci
The risks of sleeping “too much”. Survey of a National Representative Sample of 24671 adults (INPES health barometer)
PLoS One
Sleep duration and mortality: a prospective study of 113 138 middle-aged and elderly Chinese men and women
Sleep
Consequences of shiftworking on sleep duration, sleepiness, and sleep attacks
Chronobiol Int
Objective prevalence of insomnia in the general population. Sleep assessed by polysomnography in a 1042 subjects representative sample of Sao Paulo, Brazil
Ann Neurol
Computer use, sleep duration and health symptoms: a cross-sectional study of 15-year olds in three countries
Int J Public Health
Napping helps preschoolers learn
Proc Natl Acad Sci U S A
Aging and nocturnal driving: better with coffee or a nap? A randomized study
Sleep
The use of prophylactic naps and caffeine to maintain performance during a continuous operation
Ergonomics
Caffeine eliminates psychomotor vigilance deficits from sleep inertia
Sleep
Caffeine gum minimizes sleep inertia
Percept Mot Ski
Actigraphy suggests age-related differences in napping and nocturnal sleep
J Sleep Res
Effects of afternoon “siesta” naps on sleep, alertness, performance, and circadian rhythms in the elderly
Sleep
International Classification of Sleep Disorders
Actigraphic assessment of a polysomnographic-recorded nap: a validation study
J Sleep Res
Sleep duration from infancy to adolescence: reference values and generational trends
Pediatrics
Naps in children 6 months-7 years
Sleep
Sleep ontogenesis revisited: a longitudinal 24-hour polygraphic study on 15 normal infants during the first two years of life
Sleep
Genetic and environmental influences on daytime and night-time sleep duration in early childhood
Pediatrics
Genetic and environmental factors shape infant sleep patterns: a study of 18-month-old twins
Pediatrics
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- ∗
The most important references are denoted by an asterisk.