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The reason why you feel headache or nauseous or throw up is dehydration.  That is why you need to drink plenty of water before and after class.  It will take about 4-5 classes to get used to the heat.  The class is always challenging and vigorous, but its the after class feeling that makes people coming back.  You feel energized, revitalize after class, your body detoxify and your skin feels more smooth.