The science of 'Hot Yoga' (and why we do what we do!)

At the time of writing this article, it is almost impossible to find any true scientific research that backs up the claims professed by hot yoga junkies. Of the few that meet criteria for a reliable study, the results often show that the benefits of yoga in a hot and humid environment are not what they claim to be. This article instead collates information from broader research looking at the effects of exercising in heat, yoga, and the specific benefits of certain types of heat.


But first, the basics;

The human body rests at one core temperature called the “normothermia", which is usually around 36.8 degrees celsius, although of course there are inter-person variations. Our resting temperature will vary slightly throughout the day and the process of our circadian rhythms, however a variation of more than 1 degree is considered abnormal. For the body to maintain its own temperature without any aid (this includes clothes!) the external environment needs to be around 27 degrees.


If our environment begins to cool down then our body works to heat the surface of the skin up so that our internal temperature is unaffected. Likewise, if the environment begins to warm up then our body activates different mechanisms to begin cooling the surface of the skin. 


It is important to remember that our circadian rhythms evolved in line with our environments. There’s a reason some countries have siestas, and others start the working day at 3 in the afternoon. Also, one reason a morning or evening practice is often carried out early in the day is to avoid the heat inherent in the countries yoga began to evolve. Nowadays, we have so many facilities in place to moderate our environments that it is easy to lose track of these considerations (especially in the UK!), however, there is evidence to suggest that if our environment is constantly a little too comfortable (and especially, such that our body never needs to work to create and maintain heat) then our internal controls weaken and we develop a lower resting body temperature reliant on external input. This can lead to inefficient enzyme and metabolic function and a weakened digestive and immune system.


— So. Point 1. We don’t want to overrule our bodies natural ability to thermoregulate. 


At YogaTherapies, our practices use mostly static holds, which means that isometrically contracted muscles are creating stability in order to allow other specific areas of the body to lengthen. This action by itself works through a series of chemical reactions that provide the active muscles with energy, but also, the by product heat. As muscle activation increases so too does the amount of energy required and therefore more chemical reactions provide more heat. When internal heat rises, the heart has to work harder to dilate blood vessels in the skin and you begin to sweat to cool the body down. It is important to note, that the cooling response occurs through convection as the sweat evaporates. 


If the room is hot enough, or more importantly humid enough that sweat is unable to evaporate your internal body temperature will soar and potentially become severely disrupted. Our only response to try cool the body down is to increase heart rate further, in the hope that more surface blood will lose heat through convection, but this often fails to prevent core temperature rising beyond the normal level and further this heavy sweating results in dehyrdation, and decreased blood pressure. You may end up feeling weak, dizzy, crampy and nauseated (more on this later). 


For every one degree increase in core temperature a typical heart rate will increase by 30 beats per minute. Heat is a stress on our cardiovascular system (even before we factor in the exercise component!). And just to show how small our healthy margin is: once our body temperature raises beyond 39 degrees our cells and enzymes being to degrade, and ultimately our organs begin to shut down. Our gut wall also becomes more permeable, which can allow harmful bacteria to enter our bloodstream. 


How humidity affects our perception of temperature:

The temperature that a thermometer reflects rarely matches up to what we feel, which is referred to as the “apparent temperature”, which is greatly affected by humidity. In a room with 0% humidity, the temperature is likely to feel 5 or so degrees cooler than it really is, whereas in a room with 80% humidity then the temperature will often feel 5 or so degrees warmer. If you are lucky enough to be practising outside then other factors such as breeze, and relative amounts of direct sun light will also play a large role. 


So if our body knows how to regulate itself… and to externally heat the body potentially causes serious problems, why do we do it?


First, lets clear up some myths. Here are some commonly cited benefits of hot yoga that simply aren’t true:


Myth 1- You’ll lose more weight.

Research led by Emily Quandt looked in particular at Bikram style hot yoga classes. The defining characteristics being an unventilated room, heated to between 30 and 40 degrees celsius with high humidity. They found that on average, men burned 460 calories, and women 330 within a 60 minute session. This is roughly equivalent to the calorie burn of walking briskly for the same time period. However, she noted, that the heart rates measured in the yoga classes were dangerously high for such a relatively small calorie burn and cardiovascular training component.  


Myth 2 - You can sweat out toxins. 

The idea that sweating will detoxify the body on anything other than a superficial level lacks any scientific backing. As the sweat begins to pour along with a lot of water, you are losing vital minerals and electrolytes such as potassium, magnesium and sodium along with a little ammonia and urea. True toxin detoxification only really occurs in the kidneys and liver. Some research suggests that sweating accounts for no more than 2% of overall body detoxification.


Plus… if you do become very dehydrated and drink too much water (without minerals or electrolytes) during or after, you risk dangerously low blood sodium levels (or to use its proper name hyponatremia).


Myth 3 - It can make you more flexible.

There is a difference between muscular and joint flexibility, however, increasing blood flow to either area will generally “loosen” both. Ligaments typically do not require a great deal of circulation for their role creating stability. However, the hot environment often gives an illusion of increased flexibility as the resistance these areas normally hold lessens. 


The more you push into a stretch that is heavy on the joints the more likely you are to overstretch and irrevocably damage and destabilise the area. We recommend that in any posture where the stretch is felt close to or around major joints and not in the the belly of the muscle, that you ease off to the point where you can enjoy the stretch in the belly of the muscle.


Ironically, increased joint instability will often create hypertensive muscles as they attempt to compensate for the lack of stability in the ligaments by greater levels of contraction. The knees, sacroilliac joints and hamstrings are particularly vulnerable areas.


Perhaps even more cause for concern; over stretching the ligaments and over connective tissue around the joints can lead to a weakened elasticity in the fibres which causes blood to pool in the lower limbs. This further increases strain on the heart and leads to large amounts of adrenaline and endorphins being pumped into the body in an attempt to re balance the stress the body is placed under. This sensation can be experienced almost as a “high”, sometimes referred to as exertion exhilaration, and risks encouraging practitioners to repeatedly take themselves to extremes that are damaging their bodies for the experience of the adrenaline release.


And as a final point, if you cool down too quickly (which happens easily if stepping straight out into the Northern air!) then the rapid cooling causes rapid constriction of blood vessels and significant contraction of muscle fibres. There is further evidence to suggest that extreme temperature swings (and remember, it only takes 3 and a bit degrees of core temperature change before our whole body begins to fail!) can weaken the immune system. 


Myth 4 - Hot humid air is good for the lungs. 

Now, this claim has truth to it… but not in the context of a yoga class! (For respiratory benefits, go find yourself a nice sauna and steam room!)

Hot, humid air is the perfect breeding ground for a whole variety of bacteria, bugs and mould! Some styles of yoga even suggest a carpet is essential for the hot yoga environment which is even more of a health hazard! 


— Point 2. A lot of the claims are false, and even dangerously so! Fashion runs faster than the evidence can keep up with it, always be wary of people making grand claims without being able to back themselves up. 



So… What are the real benefits?

With everything, the secret to successful application is in the details! (All yoga sequencing and stylistic decisions to one side) It turns out, that there are some very real benefits to a heated yoga class, but it really depends on how you heat your space! 


Not all types of heating were created equal.


We are all well aware of the damning research looking at the over use of air conditioning and heating systems in homes and offices. Sadly, none of these will pop up if you google the risks of hot yoga. Perhaps its a problem with the way scientific literature is published and made accessible these days, but often obvious conclusions simply aren’t put together, and rather than knowledge cumulating for the common good, it is often manipulated for specific benefit. Fortunately, the therapeutic benefits of heat (completely separate from the yoga world) are well researched and evidenced.


Infrared is a band of light that we can't see but instead perceive as heat through a process called conversion. Leaving all complicated science to one side (references are included if you want to go into more depth)... You know the feeling of the sun warming you right from the inside out on a summers day? Well that's infrared! It works by vibrating the water molecules that make up 90% of us. Infrared wavelengths are easily and naturally absorbed to heat up organic substrates (like you and me) without heating the air. This means that you get lovely and hot and all glowing from the inside out, whilst the air remains cool, fresh and a delight to breathe! 


Additional benefits of infrared heating:

  • There are no emissions. Absolutely none.  
  • It actually cleans the air, and effectively stops mould. Firstly, by discouraging condensation and other factors that create an optimal environment for spores and bacteria. Secondly it actually kills many common, but dangerous bacteria and fungi. As it doesn’t work by creating a current of air, it also doesn’t encourage the movement of potentially dangerous particles or allergens… or even the circulation of odours!
  • Unlike all other forms of heating, It doesn’t effect oxygen levels in the room…meaning your inhale really will be filled with enriching oxygen!


And there’s more!

Infrared heat therapy effectively increases blood circulation without putting strain on your heart. This means higher levels of oxygen and white blood cells in your system. It also stimulates the production of collagen (a building block for human tissue) in your body and helps to rid your body of toxins through its vasodilating effects supporting effective eliminative in the kidneys and liver. The result? A stronger immune system, better cardiovascular health, and a faster ability to heal injuries. 


Different wavelengths of infrared light have also been repeatedly proven effective ways to treat arthritic and inflammatory joint and deep muscle pain.



Most importantly, why do we do what we do?

We structured our studio so that the heat of the room supports the building of heat in your practice, but does not overtake it. The level of heat also varies throughout the year, when its hot outside, we let the top limit creep a little higher, however, when you are stepping out onto a 3 degree pavement, we begin to cool the room gradually with the cooling down of the practice to ensure homeostasis is always maintained. If at all possible in the summer we love to get outside and really feel the benefits of infrared red light from its main source…the sun!


Our studio is well ventilated to ensure that the room maintains a low humidity. We heat the room on average from between 24 degrees (for our standard sessions) and 30 degrees (for our hot sessions). This means you will be receiving the maximal benefits of the heating whilst ensuring that your body remains in control, and no unnecessary stress is placed on the system. Furthermore, you won’t feel oppressed or unbearably hot on arrival and you won’t be in a puddle of your own making for savasana. 


Because of the nature of the infrared heaters we are able to zone our classroom. This means that every session has “hot spots”, and also some nice cool spots for those that prefer it. This also means that individuals who would be placed at risk within a flat temperature room such as pregnant women, or individuals with blood and heart conditions can still participate to the level that is beneficial to them! 




Finally a few tips to ensure your optimal experience:

  • Be careful about eating. Make sure you’ve had time to digest any major meals, but also, make sure you aren’t starving on arrival!  Something simple like a banana 30 minutes or so before class is a perfect top up. 
  • Avoid drinking coffee before class (or anything else that would dehydrate you for that matter!) 
  • Drink plenty of water during your usual day, and have water with you at class. If your body is well hydrated you shouldn’t be feeling overly thirsty in a 60-90 minute session. To cater for the electrolytes you are likely to loose add a pinch of organic, rock salt (we like celtic salt or himalayan pink salt) and a squeeze of lemon to your water. 
  • If you have a history of irregular blood pressure or heart disease you should definitely talk to your doctor and your teacher before you sign up.





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