What changes during the first two weeks of heat exposure
Repeated exercise in the heat triggers a set of adaptations that unfold over roughly 10 to 14 days. Plasma volume expands early, often within the first week, which improves cardiovascular stability and helps the body shed heat. Sweating begins earlier and at a higher rate, and the sweat itself becomes more dilute as the glands conserve sodium. Heart rate at a given workload falls, and core temperature at that workload tends to be lower.
Most of the cardiovascular gains appear in the first several days, while the sweat and sodium-conserving changes continue to develop across the full two-week window.
Why acclimatization fades and needs maintenance
These adaptations are reversible. After stopping heat exposure, much of the benefit decays over one to several weeks, so athletes traveling to a hot event usually re-acclimatize close to the date. Sessions of around 60 to 90 minutes of heat stress per day are commonly used, but individual responses vary widely.
This is general educational information, not medical advice. Heat training carries real risk of heat illness, and anyone with cardiovascular conditions or who is new to the heat should progress gradually and consult a qualified professional.