Napping during the night shift and recovery after work among hospital nurses1

OBJECTIVE: To analyze the association between the length of napping during the night shift and the recovery after work among nurses. METHOD: Cross-sectional epidemiological study involving 1940 nurses from 18 public hospitals in the City of Rio de Janeiro. A multidimensional and self-applied questionnaire was used with information about health, sociodemographic and occupational characteristics, health-related behaviors and housework. Multiple logistic regression was applied to identify the association, adjusted for confounding variables. RESULTS: The gross analyses showed 44%, 127% and 66% higher chances of a high level of recovery after work for nurses who sleep up to two hours, between 2.1 and 3 hours and 3.1 hours or more, respectively, when compared to the nurses who do not sleep. After adjusting for confounding variables, the association only continues significant for the group that sleeps 2.1 to 3 hours during the night shift (OR=1.79; 95%CI=1.33-2.41). CONCLUSION: The association between the length of napping and the high level of recovery after work, confirmed in the present results, can be included in the studies that aim to support more appropriate policies aimed at improving the workers' work, life and health conditions, not only in nursing, but night-shift workers in general.


Introduction
Work produces effort that reflects in emotional, cognitive and behavioral symptoms, which take some time to revert (1) . With appropriate time and sufficient possibilities of recovery (during and after work), the return to work will happen without the residual symptoms of previous efforts. When the recovery is insufficient and cumulative, however, the need for recovery increases and is gradually replaced by more severe symptoms, related to long-term fatigue and illness (2) . The need for recovery after work is characterized as an intermediary position between the demands of work and the future health condition and is related to the initial phase of the continuing process that can lead to chronic fatigue (3) .
Hospitals function uninterruptedly, which implies work schemes in day and night shifts. An essential health aspect that needs to be analyzed with regard to shift workers, especially in the night shift, refers to the influence of the work hours on the quality and length of sleep (4) and, in turn, on the recovery after work (5) .
Sleep, in terms of quality and length, varies at different times during 24 hours. Daytime sleep, usually adopted by nighttime workers, tends to be worse and shorter (6) when compared to nighttime sleep.
In that perspective, the reduced length and sleep quality changes in nursing professionals working night shifts have been demonstrated in Brazilian (6)(7) and international studies (4) . The reduced length of sleep and, more severely, the deprival from sleep entail greater risks of injuries and occupational accidents, as well as harm for these workers' health and quality of life (7) , in function of the greater sleepiness at work and reduced state of alertness deriving from nighttime work (6) .
One way to cope with this partial deprival from sleep is to nap during nighttime work (8) . Defined as any sleep period shorter than 50% of the average length of nighttime sleep (9) , napping during the night shift is associated with lower levels of sleepiness during work (8) , maintenance of alertness and performance (10) and can compensate for the short length of sleep at home among nighttime workers (11) .
In a context of nursing workers, predominantly female, nighttime naps can be even more relevant for workers with a housework overload, which implies less time for daytime sleep during the days off (12) . In addition, the 12-hour-work schemes followed by 36 or 60 hours of rest (12)(13) , which allow these professionals to be active in more than one productive activity, as well as the long workdays of the Brazilian nursing professionals, can interfere in the time available for sleep (12) .
The importance of napping during the night shift for the recovery after work has been demonstrated in a study in the context of Brazilian nursing (13), which identified the association between napping and better recovery, provided that the work journey at home does not exceed ten hours per week. This study intends to complement research on the importance of napping during nighttime work for the recovery after work in an expanded sample and considering a heterogeneous population in terms of sex.
The objective in this article was to analyze the association between the length of napping during the night shift and the recovery after work among nurses working at the main public hospitals in Rio de Janeiro.

Description of study design and sample
This article uses data from the Nurses' Health Study. A cross-sectional epidemiological study was undertaken in the 18 largest public hospitals in the city of Rio de Janeiro, RJ. The data were collected between 2010 and 2011, through the application of a structured, multidimensional and self-applied questionnaire. The 18 largest public hospitals in the city of Rio de Janeiro, under federal, state and municipal management, took part in the study. These hospitals were visited, scheduling meetings with the nursing heads to disseminate the study. The list of the name and work sector was requested for the purpose of a face-to-face approach.
The nurses received the questionnaires, together with an invitation letter to participate and the Informed Consent  Exposure variable: length of napping during night shift This variable was assessed through the following question: "As regards the time to sleep or rest during the shift, would you say that, in most cases: "you only rest"; "you sleep for about ___h___min"; "you neither sleep nor rest". The variable was categorized in four levels: (i)"does not sleep" (reference category), (ii)"sleeps up to 2 hours", (iii)"sleeps between 2.1 and 3 hours" and (iv)"sleeps 3.1 hours or more". The groups that reported that they "neither sleep nor rest" and "only rest" were included under the category of the nurses who do not sleep.

Outcome variable: recovery after work
The recovery after work was assessed using the short version of the scale of Need for Recovery -NFR.
This instrument assesses short-term effects of work induced by fatigue, such as lack of attention, irritability, social isolation, reduced performance and quality of recovery time after work (1,3) . The original scale, which consists of eleven items, was adapted to Brazilian Portuguese (14) . In this study, the short version of the scale was used, recently proposed by the author of the scale and forwarded through personal communication.
This version consists of six items from the original version with Likert-style answers (1)(2)(3)(4), with scores ranging between "always" and "never", which include fatigue-related aspects after a workday. The items were grouped in a score that was obtained by adding up the scores obtained on the set of scale items, which ranged between 6 and 24. The score was categorized in recovery tertiles, with scores 6-13 being considered as low recovery; scores 14-17 medium recovery and scores 18-24 high recovery. After this categorization, the medium and high categories were grouped, resulting in only two categories: low (reference category) and high recovery (grouping the medium and high scores).
The internal consistency, which indicates whether the items represent a shared theoretical construct, is frequently verified using Cronbach's alpha coefficient as a reliability measure (15) . In this study, however, the   Table 2 below.
An association was identified between napping during the night shift and the recovery after work.
Higher frequencies (82.5%) of high recovery after work were observed among participants who sleep between 2.1 and three hours during the night shift. The group that indicated sleeping more than three hours showed a lower recovery frequency (78.6%) than the group that indicated sleeping between 2.1 and three hours.

Discussion
In this study, napping during the night shift for 2.1 to three hours was associated with greater recovery after work. After adjusting for confounding variables, the association with other classifications of napping time did not continue. Similar results were identified in a study of female nursing professionals, only considering those submitted to short durations of housework (12)(13) .
The present study results suggest a possible beneficial effect of nighttime napping for two to three hours in terms of recovery after work. The aspects of sleep inertia should be taken into account though, a feeling of lethargy and transitory reduction of cognitive and motor performance that can occur when awaking in function of the previous waking time, the moment of the nap and its length (8,16) . In fact, this aspect deserves further analysis, as it can constitute a limiting factor for the implementation of this practice in the context of healthcare.
In this research, naps shorter than two hours seem to be insufficient to guarantee the recovery after work, possibly due to the high workload these workers are submitted to. Our results indicated a mean weekly workload of 61 hours. The extensive work burden in the context of Brazilian nursing workers has been demonstrated (13,17) and is much higher than that observed among nurses from different European countries (18) .
High burdens can affect both the workers' health and the quality of care delivery (19) .
Not only the occurrence of napping influences the recovery after work, but also its length (13) . The mean length of napping during the night shift was about two hours and a half (159 minutes), that is, higher than that identified in other studies (6,(11)(12) , which identified means of 120, 138, 141 and 150 minutes, respectively. The high weekly loading among the workers studied here may partially explain the longer length of napping. In view of the possibility of napping during the night shift, the nursing professionals get organized in function of the work demand and the preference of each (12,20) . In general, the workers themselves organize the napping times themselves, which are divided in two periods, the first from 00:00h till 03:00h and the second from 03:00h till 06:00h (6,12,20) .
Among the workers who mentioned sleeping more than three hours, no higher recovery ratios were observed when compared to those who did not sleep. viewpoint, as this practice is not formalized, restrictions existed with regard to its occurrence (20) .  (1,14) and none of the studies identified assessed the dimensional validity of the scale to permit comparisons of the presented results. Other assessments to confirm these results should be performed in other studies.

Conclusion
The association between 2.1 to three hours of napping and the high recovery after work was confirmed