Bathing 1-2 hours before bedtime can significantly improve your sleep

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Biomedical engineers at The University of Texas at Austin may have found a way for people to get better shuteye.

Systematic review protocols – a method used to search for and analyze relevant data – allowed researchers to analyze thousands of studies linking water-based passive body heating, or bathing and showering with warm/hot water, with improved sleep quality.

Researchers in the Cockrell School of Engineering found that bathing 1-2 hours before bedtime in water of about 104-109 degrees Fahrenheit can significantly improve your sleep.

“When we looked through all known studies, we noticed significant disparities in terms of the approaches and findings,” said Shahab Haghayegh, a Ph.D. candidate in the Department of Biomedical Engineering and lead author on the paper.

“The only way to make an accurate determination of whether sleep can in fact be improved was to combine all the past data and look at it through a new lens.”

The paper explaining their method was recently published in the journal Sleep Medicine Reviews.

In collaboration with the UT Health Science Center at Houston and the University of Southern California, the UT researchers reviewed 5,322 studies.

They extracted pertinent information from publications meeting predefined inclusion and exclusion criteria to explore the effects of water-based passive body heating on a number of sleep-related conditions: sleep onset latency – the length of time it takes to accomplish the transition from full wakefulness to sleep; total sleep time; sleep efficiency – the amount of time spent asleep relative to the total amount of time spent in bed intended for sleep; and subjective sleep quality.

Meta-analytical tools were then used to assess the consistency between relevant studies and showed that an optimum temperature of between 104 and 109 degrees Fahrenheit improved overall sleep quality.

When scheduled 1-2 hours before bedtime, it can also hasten the speed of falling asleep by an average of 10 minutes.

Much of the science to support links between water-based body heating and improved sleep is already well-established.

For example, it is understood that both sleep and our body’s core temperature are regulated by a circadian clock located within the brain’s hypothalamus that drives the 24-hour patterns of many biological processes, including sleep and wakefulness.

Body temperature, which is involved in the regulation of the sleep/wake cycle, exhibits a circadian cycle, being 2-3 degrees Fahrenheit higher in the late afternoon/early evening than during sleep, when it is the lowest.

The average person’s circadian cycle is characterized by a reduction in core body temperature of about 0.5 to 1 F around an hour before usual sleep time, dropping to its lowest level between the middle and later span of nighttime sleep.

It then begins to rise, acting as a kind of a biological alarm clock wake-up signal.

The temperature cycle leads the sleep cycle and is an essential factor in achieving rapid sleep onset and high efficiency sleep.

The researchers found the optimal timing of bathing for cooling down of core body temperature in order to improve sleep quality is about 90 minutes before going to bed.

Warm baths and showers stimulate the body’s thermoregulatory system, causing a marked increase in the circulation of blood from the internal core of the body to the peripheral sites of the hands and feet, resulting in efficient removal of body heat and decline in body temperature.

Therefore, if baths are taken at the right biological time – 1-2 hours before bedtime – they will aid the natural circadian process and increase one’s chances of not only falling asleep quickly but also of experiencing better quality sleep.

The research team is now working with UT’s Office of Technology Commercialization in the hopes of designing a commercially viable bed system with UT-patented Selective Thermal Stimulation technology.

It allows thermoregulatory function to be manipulated on demand and dual temperature zone temperature control that can be tailored to maintain an individual’s optimum temperatures throughout the night.


Insomnia is a common complaint in older adults.

They have trouble falling asleep, as well as frequent or prolonged nocturnal or early morning awakenings with an inability to return to sleep (Ancoli-Israel, 1997Ancoli-Israel & Roth, 1999Floyd, Medler, Ager, & Janisse, 2000).

The overall prevalence of insomnia in the elderly ranges from 14.0% – 42.2% based on epidemiological surveys (Chiu et al., 1999Kim, Uchiyama, Okawa, Liu, & Ogihara, 2000Ohayon, 1996).

As many as 25% of healthy elderly men and women may have chronic insomnia (Ohayon & Smirne, 2002).

Global sleep dissatisfaction, defined as overall dissatisfaction with sleep, ranges from 7.7 – 20.8% in the elderly, which is higher than in the general population (7.0-10.1%; Pallesen et al., 2001).

Sleep is periodic resting behavior characterized by “few body movements, a recumbent posture, and complex brain electroencephalographic activity” (Carskadon & Dement, 2000, p.15).

Human sleep architecture is defined by polysomnography (PSG) and divided into stages: four of non-rapid eye movement (NREM) sleep (stage 1 to 4), and one stage of rapid-eye movement (REM) sleep (Rechtschaffen & Kales 1968).

Stage 1 is a transitional stage between waking and other sleep stages. Stage 2 is characterized by periodic sleep spindles and comprises most NREM sleep.

Stages 3 and 4 referred to as slow wave sleep (SWS) are defined by higher amplitude and slower electroencephalogram (EEG) frequency activity (delta wave).

In a young adult, a typical night of sleep has 4 to 5 periods of NREM and REM sleep (approximately 90-120 minutes per period) and minimal wakefulness (Carskadon & Dement; Culebras, 2002).

PSG is the most valid and accurate way to assess sleep.

Measures derived from PSG include: (a) total sleep time, (b) sleep efficiency (ratio of time spent asleep/time in bed), (c) sleep latency (time to fall asleep after lights out), (d) amount of wake time during sleep periods (waking after sleep onset, WASO), (e) number of awakenings, and (f) amount of each sleep stage (Landis, 2002).

Diagnostic criteria for insomnia include a sleep latency or amount of WASO > 30 minutes and a sleep efficiency less than 85% of time in bed, occurring ≥ 3 nights per week for at least 1 month (Edinger et al., 2004).

Age has large effects on sleep architecture. Compared to young adults, PSG-derived sleep architecture in older adults shows increased amounts of nocturnal wakefulness, increased NREM stage 1 sleep with reduced amounts or complete absence of NREM stages 3 and 4 sleep (Floyd et al., 2000Van Someren, 2000a).

In some studies, the percentage of REM sleep was reduced slightly in older adults (Van Cauter, Leproult, & Plat, 2000), whereas findings from other studies showed that older adults experienced typical amounts of REM sleep (Campbell & Murphy, 1998).

Long sleep latencies (e.g. > 30 minutes), reduced sleep efficiency (e.g. < 85%), and a sleep duration of ≤ 6 hours were common in older adults.

These characteristics are consistent with the criteria of insomnia, although not all older adults complained of poor sleep (Vitiello, Larsen, & Moe, 2004).

In addition to changes in sleep architecture, there are changes in the circadian rhythm of body temperature in older adults compared to young adults.

A circadian rhythm is the variation in body functions and behaviors that occur in a regular pattern around a 24 hour time interval (Aronson et al., 1993Aschoff, 1984).

Body temperature variation and the sleep wake cycle each follow a circadian rhythm. These rhythms are coupled or synchronized with each other in people who are active in the day and sleep at night.

In the morning after waking up, core (rectal) body temperature rises throughout the day to reach its highest point (peak) in the afternoon or early evening.

In the evening, body temperature declines prior to sleep onset and continues to fall during sleep to reach its lowest point (trough) in the early morning hours (Dijk & Czeisler, 1995Lavie, 2001Van Someren, Raymann, Scherder, Daanen, & Swaab, 2002).

The difference between the peak and trough of a circadian rhythm is called the amplitude. Compared to young adults, the circadian rhythm of core body temperature in older adults shows decreased amplitude (Dijk, Duffy, & Czeisler, 2000Van Someren, 2000a).

This difference could result from a lower daytime peak, a higher nighttime trough, or a combination of both. Figure 1 shows the circadian core body temperature rhythm in young and older adults.

The trough temperature during sleep is higher and the peak is lower in older adults compared to young adults (Carrier, Monk, Buysse, & Kupfer, 1996).

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Figure 1

The fall in core body temperature before sleep onset and during sleep is associated with dilation of peripheral blood vessels, permitting heat dissipation from the body core to the periphery (Krauchi, 2002Krauchi, Cajochen, Werth, & Wirz-Justice, 2000Krauchi & Wirz-Justice, 2001).

A lower core (rectal) temperature coupled with a higher distal (hands and feet) temperature before sleep is associated with a shorter sleep latency (Krauchi, Cajochen, & Wirz-Justice, 2004).

The difference between core and peripheral temperatures is reflected by the distal-proximal skin temperature gradient (DPG) and has been shown to be a good predictor of sleepiness and the body’s readiness for sleep (Krauchi, Cajochen, Werth, & Wirz-Justice, 1999).

Treatments that enhance the peripheral gradient for heat loss have the potential to facilitate sleep onset and quality.

Based on evidence of the relation between body temperature and sleep, a warm bath (passive body heating) in the evening may promote sleep onset and improve overall sleep quality in older adults.

Previous researchers have found that compared to a non-bathing night, a warm bath (40∼ 41°C) with whole body immersion in a bathtub taken 2 to 4 hours prior to sleep, shortened sleep latency, decreased the amount of wake time after sleep onset, and increased the amount of slow wave sleep, but also raised core (rectal) body temperature in older women with insomnia (Dorsey et al., 1996Dorsey et al., 1999).

Compared to whole body immersion in a tub, a warm bath (42°C) of only the lower legs taken within an hour of sleep onset improved sleep, but did not raise core body temperature in young women (Sung & Tochihara, 2000).

A warm foot bath may increase peripheral blood flow and temperature gradient (DPG) to facilitate heat loss without increasing core body temperature and hereby improve sleep onset and quality.

This hypothesis had not been tested in older adults with sleep disturbance.

The purpose of this single group, experimental crossover design study was to examine the effects of a warm footbath prior to sleep onset on body temperature and sleep in community dwelling older adults complaining of sleep disturbance.

The conceptual framework for this study is shown in Figure 2.

A warm foot bath was thought to facilitate heat dissipation to lower core (rectal) body and abdominal temperatures and raise foot temperature, and simultaneously to improve objective and perceived sleep outcomes.

We hypothesized that a warm foot bath would raise the peripheral gradient for heat loss (increased DPG), improve PSG and perceived sleep patterns (increase total sleep time, shorten sleep latency, raise sleep efficiency, reduce WASO and number of awakenings, increase SWS), and improve perceived sleep quality (raise overall quality, improve sleep restoration and satisfaction) in older adults with complaints of sleep disturbance.

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Figure 2

More information: Shahab Haghayegh et al. Before-bedtime passive body heating by warm shower or bath to improve sleep: A systematic review and meta-analysis, Sleep Medicine Reviews (2019). DOI: 10.1016/j.smrv.2019.04.008

Provided by University of Texas at Austin

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