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An Introduction to Sauna Physiology

homeodynamics of stress

Guest post series continues. Please welcome Hannes Zedel. Hannes is an interdisciplinary scientist with his primary background in neuroscience. He is reputed for breaking down the physiology of the Wim Hof Method and has done further work in the fields of psychology, environmental science, microelectronics, material science, data analysis, and modeling. Enter Hannes:

Summary/Best practice recommendations

  • Drink about half a liter of water, ideally with mineral salt
    On average, one loses about 0.5kg of sweat in every ~20-minute sauna session. The water and electrolytes should immediately be replenished. About 1-2 teaspoons of salt per liter of water is a good rule of thumb. People with poor blood sugar regulation may consider adding some kind of sugar in small amounts.
  • Shower first
    Being clean before entering the sauna protects oneself, others, and the wood in the sauna.
  • Enter dry
    As some of the benefits are mediated by sweating and having wet skin from showering delays sweating, one should get dry before entering the sauna.
  • Sit on a towel (hygiene, wood preservation)
    Similar to the prior shower, this contributes to hygiene and preserves the wood of the sauna.
  • Infusion once one starts sweating
    As the skin develops a wet film from sweating, one can intensify the session with an infusion (pouring water over the hot stones).
  • Aim for about 20 minutes per session
    Evidence suggests that the duration of each session affects some of the benefits and about 20 minutes was found to be most effective on average. As many factors affect the ideal duration, this only serves as a rough orientation. Furthermore, one should gradually build towards longer durations to adapt to the heat stress.
  • Take a cold shower or bath afterwards.
    Aim to perceive mild cold pain / a constricted feeling of the blood vessels (rule of thumb: 0.5 to 3 minutes). The cold exposure addition makes blood vessels cover their whole range of motion and thereby adds to cardiovascular benefits. It also interrupts momentary heat adaptation mechanisms and thereby prevents unnecessary metabolic exhaustion.
  • Drink & Rest
    Rest for about half as long as the prior heat exposure or longer (10 minutes+).

Hormetic stress in sauna use (more is not better):

Proper use of sauna involves exposure to physiological stressors such as heat, cold and subsequent metabolic challenges. The overall effects of such stressors are typically dose-dependent and follow a characteristic curve as outlined in Figure 1. 

Figure 1: The direction of the effect of a hormetic stressor depends on the stressor’s dose/intensity. Adapted from: https://sureshrattan.com/category/researchs/c37-hormesis/

The related dose-dependency concept is called hormesis and intuitively explained by the example of physical exercise: With regard to an individual’s current level of training, exercising too little will have no effect while exercising too much will cause injuries. In between is the so-called “hormetic zone/range” of stimulus intensity that induces beneficial adaptations without causing adverse effects. It is a fundamental concept that explains how the human body adapts to its environment and applies to a wide range of physiological stressors. Training progressions relate to the expansion of the hormetic range as the body gradually adapts to the stimuli.

Inducing stress within the hormetic range implies a critical differentiation to chronic stress, which accounts for the general negative connotation of stress. It also explains, against common belief, how more is not always better. The hormetic range of a physiological stressor is constantly changing and shifting depending on many lifestyle factors so one has to continually adapt exposure to stressors – be it physical exercise or thermal stress in the use of sauna.

The sensory side: Thermoreception

Sauna primarily induces thermal stress – heat exposure during the sauna session and cold exposure afterwards by using a cold shower or bath. Related responses are mediated by different types of thermoreceptors that can be classified by their responses to different temperature ranges. A simplistic overview of those responses is shown in Figure 2.

Figure 2: Simplified overview of thermoceptor responses and corresponding classification. From: https://steemit.com/sirwinchester/@sirwinchester/til-how-can-water-be-so-hot-that-it-feels-cold originally adapted from https://www.researchgate.net/figure/Discharge-frequencies-at-different-skin-temperatures-of-thermoreceptors-along-with_fi

Peripheral thermoreceptors (skin, extremities) may contribute to other components of the overall range of effects than central thermoreceptors (core temperature). There are a couple of things to note about the functioning of thermoreceptors. First of all, the effective temperature does not only depend on the environmental temperature but also other environmental conditions such as air humidity, air flow dynamics (wind), and air pressure. Furthermore, those receptors respond primarily to a certain change in temperature and constantly retune themselves, which means a sudden temperature change elicits different responses than a slow gradual change. This has implications for different sauna practices such as different types of sauna infusions. Notably, thermal thresholds as in the figure above or other studies are usually referring to relatively static environments in standard conditions and slow temperature changes. Therefore, those thresholds vary considerably in real environments and looking at the temperature alone is widely insufficient.

Therapeutic thermal stress stimulates thermoreceptors below ~16C or above ~44C (at otherwise standard conditions) and induce a pain sensation rather than a thermal sensation, which is why people can often not differentiate between heat and cold under some circumstances such as contrast showers. The activation of those receptors is associated to many of the hormetic responses and desirable effects on inflammation, cardiovascular health, mood, and others.

Practically, mild heat stress already mediates some benefits via sweating and the cardiovascular stimulation, however, extending the session to stimulate the painful range of heat stress to some degree widens the range of effects, covering more pronounced hormonal responses and other benefits. Those responses are mostly rather binary, meaning it is usually not worthwhile to push the duration in that state as one would raise the intensity of the session beyond the hormetic range very quickly.

Selective overview of sauna benefits

Documented effects and benefits of sauna use include:

  • Improved cardiovascular health & exercise performance
    Sauna sessions have similar effects as sessions of mild to moderate physical exercise including the wide range of associated benefits.
  • Inflammation
    Sauna use attenuates inflammation as marked by interleukins and CRP.
  • Upregulation of Heat Shock Proteins (HSPs)
    Those proteins are induced by thermal stress in either direction and contribute to protein stabilization. Simply put, HSPs serve as a means of repair and homeostasis under changing environmental conditions.
  • Depression
    Sauna induces lasting antidepressant effects, attributed to the modulated inflammatory response and hormonal responses to heat stress, involving e.g. noradrenalin, dynorphin, and others.
  • Growth hormone
    Post-exercise sauna was shown to increase growth hormone several-fold depending on the intensity of the sauna session.
  • Blood sugar
    Insulin & glucose regulation benefit from regular sauna use.
  • Heat acclimatization
    Expectedly, by sauna use the body becomes better at dealing with future heat stress.
  • Detoxification
    Some types of metabolic waste and toxins can be removed via sweating.

Disclaimers

Patients with cardiovascular, pulmonary or other related conditions should consult their health professional before using sauna.

There is limited data on the safety of sauna for pregnant women. Finnish studies and anecdotes suggest that women who are acclimatized to sauna use can probably continue their practice throughout pregnancy without concerns, however, available data is not conclusive.

Heat stress can temporarily impair male fertility; however, those impairments were shown to be reversed back to normal a few weeks after ceasing heat stress.

Children are considered more susceptible to heat stress than adults, however, as long as being supervised by adults, the participation of children in sauna practice is considered rather safe. Empirical data is lacking.

Editor’s Note: Hannes can be reached directly at : dashannes@gmail.com.

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