CLINICAL REVIEWBefore-bedtime passive body heating by warm shower or bath to improve sleep: A systematic review and meta-analysis
Introduction
Bathing has long been linked not only with cleanliness and spiritual and religious purification [1] but health preservation and rehabilitation [2]. During the Homeric era of 1200 to 800 BC bathing was viewed exclusively as hygienic [2]; however, Hippocrates (460–370 BC) regarded it as health promoting and therapeutic. The Romans also considered it to be curative, in addition to relaxing and pleasurable [2]. During the 16th century, bathing, especially in spring waters, was pursued for healing, and commencing in the 19th century spas specializing in hot and cold mineral baths, commonly combined with mud packs, physical exercise, massage, and special diets, became popular as a holistic remedy of diverse maladies [2]. Indeed, at this time warm baths were extensively extolled for their healthful benefits, being advocated, for example, in the United States by Dr. John Gunn in his popular home medical handbook, ‘Gunn's Domestic Medicine’ [3]. However, there seems to be no evidence warm or hot baths were specifically used to facilitate or improve sleep until the 20th century, as reviewed by Raymann et al. [4].
In this context, the relationship between blood distribution, body temperature, and states of sleep and wakefulness has long been of interest. The Greek philosopher Alcmeaon of Croton, ∼500 BC, associated the state of sleep with the withdrawal of blood from the periphery to the larger “blood-flowing” vessels and waking with its “rediffusion” [5], and the 7th century essayist Robert Burton wrote “A hot brain does not sleep” [6]. In the 19th century Davy and Jürgensen described the human temperature circadian rhythm characterized by elevated daytime and depressed nighttime level [7], [8], and Dalton elucidated the “normal variations of temperature in the living body”, with differences between central and peripheral sites [9]. Collectively, these theoretical conceptualizations and scientific discoveries laid the foundation for contemporary investigations substantiating association between sleep and decline at the end of the activity span of core and brain temperature achieved by circadian rhythms governing the dissipation of body heat ∗[10], ∗[11], ∗[12], ∗[13]. Accordingly, alteration of the temperature circadian rhythm, e.g., by disease, work schedule, and transmeridian travel, is commonly associated with episodic or chronic insomnia – prolonged sleep onset latency (SOL) and/or poor sleep maintenance [14], ∗[15], [16], [17], [18]. The estimated prevalence of insomnia of various etiology, including that directly or indirectly associated with perturbed temperature circadian rhythm, is substantial, being 1.5-fold higher in women than men and much higher in the elderly than young ∗[4], [19], [20], [21], [22], [23]. The consequent sleep deprivation compromises quality of life and productivity and increases vulnerability for accidents and injury [24]. Moreover, it is implicated as a cause or effect of psychiatric, cardiovascular, metabolic, and other medical conditions, whose healthcare expenditures globally amount to billions of dollars annually [19], [21], [25].
Water-based passive body heating (PBHWB) accomplished by warm/hot showers or body or foot bathing is often recommended as a simple and low-cost nonpharmacological means of managing insomnia [26], [27], [28]. Such advice largely derives from findings of questionnaire and observation-based investigations. For example, Aritake-Okada et al. [29] found bathing to be an effective means of improving nighttime sleep and decreasing excessive daytime sleepiness; Ojima et al. [30] reported good subjective sleep quality of persons who regularly bathe; Hayasaka et al. [31] detected positive relationship between frequent bathing and self-assessed sleep quality and Goto et al. [32] likewise demonstrated positive association between frequent bathing combined with onsen (hot springs) facilities and self-rated sleep quality; and Camilleri and Barrett [33] observed better sleep of seniors with late-day bathing. Findings of objective laboratory studies complement the positive effects of PBHWB inferred by subjective measures. Yu et al. [34] found footbath PBHWB more acceptable by men than women and more effective in winter than summer; Raymann et al. [35] showed skin temperature manipulation by PBHWB strongly influences amount of rapid eye movement (REM) and slow wave sleep (SWS); and Deguchi et al. [36] described immediate (after 2 wk) and more substantial long-term (after 4–6 wk) improvement in sleep, along with reduction in restlessness, wandering, and/or aggression, in 60–90% of a small cohort of senile dementia patients when bathed twice weekly in the early evening (18:00–19:00 h) than early afternoon (14:00–15:00 h). Nonetheless, while many investigations demonstrate improvement of various sleep parameters by evening or before bedtime PBHWB treatment, some do not ∗[4], [37], [38]. The goal of this article is to review the published research concerning the potential beneficial effects of PBHWB on nighttime sleep and proposed mechanisms.
Section snippets
Methods
The review was conducted in adherence with the Preferred Reporting Items for Systematic Review and Meta Analyses (PRISMA) statement [39]. The PRISMA checklist is provided in Appendix A.
Search results
The flow diagram of Fig. 1 depicts the PRISMA-based process of selecting investigations of relevance utilizing the search words depicted in Table S1. A total of 5316 publications were retrieved, of which 1451 were replicates. Six additional publications were identified through reference lists or other sources. Abstracts of the potentially relevant 3865 total publications were individually screened, yielding 121 for full text appraisal, of which 17 met all inclusion criteria. Some 13 [37], [38],
Discussion
A warm shower or foot or body bath before bedtime is often suggested by medical and paramedical professionals to improve sleep [26], [27], [28]. The beneficial effect of such interventions has been researched in various settings utilizing a variety of measures, ranging from subjective self-report questionnaires and objective check lists to wrist actigraphy/home sleep trackers to clinical PSG. Many, but not all, of the diverse investigations report significant improvement in objectively assessed
Conclusion
Our systematic review of the literature supports the conclusion nightly warm showering, foot bathing, or full body bathing scheduled 1–2 h before bedtime for a duration as short as 10 min can improve sleep, especially shortening of SOL, most likely by enhancing decrement in CBT before bedtime. Our conclusion regarding the proper scheduling of PBHWB before bedtime, however, is based on the findings of only two studies entailing in total 36 subjects, thereby highlighting the need for more
Conflicts of interest
The authors do not have any conflicts of interest to disclose.
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