Heated classes are a substantial part of the American yoga market. The heat produces real physiological changes, and several of them are commonly misattributed.
Warm tissue moves further
Raising tissue temperature reduces the stiffness of muscle and connective tissue, so the same effort produces a greater range of motion.
The change is temporary and reverses as the tissue cools, which is why range achieved in a heated room is not necessarily available the next day.
Mistaking this for improved flexibility can lead a practitioner to work at a depth their tissue is not adapted to when cool.
Sweat is not a measure of effort
Sweating is the body's response to heat load, and in a hot room it occurs whether or not the class is physically demanding.
The volume of sweat therefore says nothing about intensity, and a gentle heated class can produce more of it than a strenuous unheated one.
The commonly claimed detoxification effect is not supported; sweat is primarily water and salts, and the organs that clear waste are the liver and kidneys.
Fluid loss changes how the class feels
Substantial sweating reduces blood volume, which raises heart rate at a given workload and makes standing after floor poses more likely to produce dizziness.
Fluid and electrolytes need replacing after class, and drinking only water after very heavy sweating is worth being aware of.
Lightheadedness, nausea or the cessation of sweating during class are signals to stop and leave the room rather than to continue.
Heat masks the usual feedback
Discomfort ordinarily signals the boundary of a safe range, and the combination of heat and elevated tolerance can dull that signal.
Injuries in heated practice frequently involve moving further than the tissue was prepared for, which the room made possible without making it safe.
Experienced teachers address this by cueing effort rather than depth, and by discouraging comparison across a heated room.
Who should approach it cautiously
Heat stress affects people differently, and cardiovascular conditions, pregnancy, some medications and a history of heat illness all change the risk.
Anyone in those categories should ask a doctor before taking heated classes rather than testing tolerance in the room.
The practice is not inherently unsafe; it adds a heat load to exercise, and that load is what needs accounting for.