Last updated: March 20, 2020
One version of this concern goes like this
So don’t fool yourself into thinking that just because the warmth in the room allows you to go deeper into the pose that your deeper tissues are receiving any benefit—that feel-good stretching sensation is probably just your muscles talking. That’s the feedback we’re used to listening to because we’ve been doing it for years. But if your intention is to work on increasing your flexibility by targeting the connective tissue—a worthy goal because it complements strengthening and stretching practices and improves overall joint health—then yin in a hot room is not the way to achieve that aim.
Part of this concern is valid and I have addressed this concern more fully in the article Hot Yin. It is certainly true that more stress will reach the tendons, ligaments and deeper connective tissues when the body is cooler, but this does not mean that there is no benefit at all to stressing our tissues when the body is warm. Even with warm muscles, tendons and ligaments receive some stress. And, for many people, it is only when their body is warmed up that they can move deep enough into a posture to get a stress into their targeted tissues. In other words, some students need to be warm to get any sensation or stress.
We can generalize and say that the cooler the tissues, the more stress from a pose will be experienced by the deeper tissues; however, this does not mean that no benefit is gained by stressing a warm body in a yin way. When our tissues are warmed up their viscosity is decreased. A temperature increase in fascia of up to 105°F (~ 40°C) leads to reduced stiffness and more rapid elongation of the tissue, which in part can be attributed to higher extensibility of collagen. In other words, it is easier to get longer when the tissues are warmer. Additionally, Gerald Pollack, a water scientist, has looked at the thixotropic properties of our ground substance (water). He has shown that constant stress or heat can turn the normal gel state (bulk, solid or thick state) of our water into a sol state (which is a more liquid, flowing state of water). We need both states at various times, so switching states is quite healthy. We don’t want the ground substance to always be sol or always gel. Dr. Robert Schleip has noted that when the water in our fascia is in a more sol state, free radicals and other toxins can be more easily removed.
Additionally, the benefits from Yin Yoga are not purely physical: students also benefit from the stillness the practice offers. The opportunity to come to an edge, become still and be with sensations is still available even in a hot room. Indeed, there may be more sensations to be aware of in a hot room. The opportunity to practice mindfulness is enhanced by remaining still for 3, 5 or 10 minutes at a time. The temperature of the room need not diminish the quality of the mindfulness practice. Indeed, this is one reason many people are adding hot yin to their yoga practice—it is their chance to calm the mind and practice presence.
Another benefit is the energetic stimulation available in Yin Yoga. Whenever an edge is felt, whenever a posture challenges us physically, tissues are being stressed. These stresses create mechanical, electrical and chemical signals that travel through the body and can change the quality of the tissues that transmit these signals. In the East, this is called prana or chi, which flows through nadis and meridians. The Daoists call the mechanical stimulation acupressure. Even in a hot room, the body can experience acupressure, which stimulates energies to flow.
While it is true that more physiological benefits may be available to the student who practices in a cooler environment, it is not true to say that practicing Yin Yoga in a warm room is not healthy or beneficial. The fact that hot yin has become popular attests to the fact that people are getting benefits from the practice. Can it be too much? Is there a danger that, in a hot room, the tissues may be stressed too much? Of course! Anything can be overdone. However, this is a risk for a hot yang yoga practice more than for a hot yin practice, because the yang yoga movements are deeper and more dynamic than those used in Yin Yoga. Whether in a hot or cool room, students must always check in and see what their body is telling them, and respect those signals. Inherently, however, there is no reason to avoid hot yin—if you like being in a warm room, enjoy it! It is certainly better than not doing any yin at all.
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 See Werner Klinger, “Temperature Effects on Fascia,” in R. Schleip, T. Findley, and P. Huijing (eds.), Fascia: The Tensional Network of the Human Body (London, UK: Churchill Livingstone, 2012), 421.
 Thixotropic means that fluids or gels can become less viscous with pressure or heat.
 “In healthy fascia, a large percentage of the extracellular water is in a state of bound water (as opposed to bulk water) where its behaviour can be characterized as that of a liquid crystal (Pollack, 2001). Much pathology – such as inflammatory conditions, edemae, or the increased accumulation of free radicals and other waste products – tends to go along with a shift towards a higher percentage of bulk water within the ground substance. Recent indications by Sommer and Zhu (2008) suggest that when local connective tissue gets squeezed like a sponge and subsequently rehydrated, some of the previous bulk water zones may then be replaced by bound water molecules, which could lead to a more healthy water constitution within the ground substance.” Excerpted from Robert Schleip and Divo Gitta Müller, “Training principles for fascial connective tissues,” Journal of Bodywork & Movement Therapies (2012): 1–13.