A good idea to take a hot shower?

Key points to remember.

Hot water promotes relaxation, relaxes the muscles and may help to reduce the time it takes to fall asleep.
Above 39°C, water tends to dissolve the protective lipids, increase the skin’s pH and accelerate its dehydration.
Excessive heat dilates the blood vessels, which can worsen rosacea and skin inflammation.
Very hot water weakens the structure of the hair and can make it more porous.
It is preferable to opt for a lukewarm shower (37–38°C).

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What are the benefits of a hot shower?

Taking a shower at a comfortable temperature is an essential hygiene practice, allowing not only the removal of impurities and body odours, but also acting as a decompression ritual.

After an intense day, the warmth of the water helps to calm the mind, clarify one’s thoughts and restore a sense of serenity. From a physiological perspective, warm water has a vasodilatory effect: it dilates the blood vessels, thereby improving blood flow and tissue oxygenation. Up to 39°C, water helps with relaxation and provides an immediate sense of well-being.

Furthermore, warm water has heat-related properties that are particularly relevant for the musculoskeletal system. It relaxes muscular tension and releases contractions that have built up through fatigue or intense physical exertion. By easing stiffness in the back and joints, jets of warm water act as a natural form of therapy, especially beneficial for individuals suffering from chronic pain or muscle soreness after exercise.

Beyond muscle relaxation, hot water helps to improve sleep quality. According to a scientific review that compiled data from numerous medical databases, taking a shower at a temperature between 40 and 42.5°C promotes faster sleep onset and better sleep quality. More specifically, the heat of the water increases blood flow in the hands and feet, which promotes the dissipation of body heat and leads to a drop in core body temperature, a biological signal that induces sleep. This signal is also the reason why it is recommended to sleep in a temperate bedroom, at around 18°C.

In summary, a hot shower is a practical way to relax, ease muscle tension, and promote falling asleep.

Very hot showers: are there side effects to be concerned about?

39°C

It is generally considered that the water temperature for a shower should not exceed 39°C.

While heat provides immediate relief, an excessively high temperature can prove counterproductive for skin health.

Very hot water does indeed tend to dilate the blood vessels, which leads to the appearance of redness, particularly in people with sensitive skin or prone to rosacea. Another issue is that it tends to alter the hydrolipidic film. This natural mixture of sebum and sweat acts as a barrier on the skin’s surface, protecting the epidermis from external aggressions and limiting transepidermal water loss. It should be noted that, conversely, very cold water can also be too aggressive for the skin.

The harmful impact of heat on the skin barrier has been quantified in a study conducted with 50 volunteers. By comparing immersion of the hands in hot water (around 41°C) and cold water (around 11°C) for 10 minutes, the researchers observed that exposure to hot water significantly increased transepidermal water loss, rising from 25.75 to 58.58 g·h⁻¹·m⁻², indicating dehydration of the skin. The study also reported an increase in skin pH, making the skin less acidic and therefore more vulnerable, as well as a pronounced erythema. These findings confirm that prolonged exposure to excessively hot water can damage the skin.

Influence de la température de l'eau sur l'érythème et la perte insensible en eau (TEWL). — Influence of water temperature on erythema and transepidermal water loss (TEWL).
Source : ARIAS-SANTIAGO S. & al. Impact of water exposure and temperature changes on skin barrier function. Journal of Clinical Medicine (2022).

Finally, the effects of temperature are not limited to the epidermis and also concern the hair fibres. Research on the stability of hair components has shown that immersion in hot water causes a degradation of the biomarkers located within the fibre. According to a clinical study, the higher the temperature (from 40°C to 80°C), the faster and more substantial the loss of internal components. Cortisol, for example, is considered a reliable biomarker of chronic stress.

> 60%

of loss of capillary cortisol after prolonged immersion of hair at 40°C.

> 90%

of loss of capillary cortisol after prolonged immersion of hair at 80°C.

Note : Hair cortisol corresponds to the fraction of cortisol incorporated into the fibre during its growth. Unlike blood cortisol, which fluctuates rapidly, its presence in hair reflects cumulative and prolonged exposure to circulating cortisol. It is therefore used as a biomarker of chronic stress, as it makes it possible to trace, over several weeks to several months, the activity of the hypothalamic–pituitary–adrenal axis. Its degradation under the effect of heat is indicative of a more global alteration of the integrity of the internal components of the hair.

In addition to weakening the skin, taking a shower that is too hot can compromise the cohesion of the hair fibre and make it more porous and more fragile. Lukewarm water, at around 37°C, appears to be a good compromise for enjoying the benefits of a hot shower without compromising the skin and hair.