Heat
Ten to fourteen days of consistent heat exposure produces measurable changes in plasma volume, sweat onset, sweat composition and heart rate. It is among the most reliable adaptations in exercise physiology.
The first hot week of the year is disproportionately dangerous. Emergency presentations for heat illness cluster in early-season heat waves rather than in the hottest weeks that follow, and the difference is acclimatisation.
Plasma volume expands, typically by 3 to 15 percent, within the first three to six days. This is the earliest adaptation and it drives several others: greater blood volume supports both skin perfusion for heat dissipation and muscle perfusion for work, which previously competed.
Sweating begins at a lower core temperature. The threshold shifts downward, so cooling starts earlier in the session rather than after core temperature has already climbed.
Sweat rate increases, often substantially, and distribution across the body becomes more even.
Sweat becomes more dilute. Aldosterone-mediated sodium reabsorption in the sweat gland duct improves, and sodium concentration can fall by half or more. This conserves electrolytes and reduces cramping risk.
Heart rate at a given workload falls, commonly by 10 to 20 beats per minute, reflecting improved cardiovascular efficiency and stroke volume.
Core and skin temperature at a given workload fall, and perceived exertion falls with them.
The stimulus is elevated core temperature sustained for a meaningful period, repeated daily. The standard protocol is 60 to 90 minutes of moderate exercise in heat, once daily, for 10 to 14 consecutive days.
Intensity should be moderate rather than maximal. The goal is sustained core temperature elevation, not peak effort, and hard sessions in unaccustomed heat carry real risk before adaptation is established.
Consistency beats intensity. Daily exposure produces adaptation reliably; every-other-day exposure produces it more slowly and incompletely.
Several substitutes have supporting evidence.
Post-exercise hot water immersion — 30 to 40 minutes in water around 40°C immediately after normal training — has been shown to produce meaningful acclimatisation in athletes training in cool conditions.
Sauna use following training appears to produce similar though somewhat weaker effects.
Overdressing during training raises core temperature but impairs evaporative cooling and requires care, since it removes the safety margin that sweating normally provides.
Adaptations are not permanent. Plasma volume expansion is among the first to reverse, declining within about a week of removal from heat. Broader adaptation decays over two to four weeks, though some sweat-gland changes persist longer.
Reacquisition is faster than initial acquisition, typically taking a few days rather than two weeks. Practically, this means a week of cool weather mid-summer does not undo the season, but a two-week holiday in a cool climate meaningfully does.
It raises the ceiling; it does not remove it. Acclimatised individuals still suffer heat illness in sufficiently extreme conditions, and the improved comfort can encourage exactly the overreach that causes it.
Nor does it reduce fluid requirement. Acclimatised people sweat more, so their hourly fluid needs are typically higher, not lower. They simply cool more effectively per unit of core temperature rise.
Acclimatisation makes you better at handling heat and thirstier while doing it. Both halves of that need to be in the plan.
Begin heat exposure before you need it. Two weeks of moderate sessions in early-season warmth positions you far better for the first heat wave than arriving unadapted.
Treat the first hot week of the year, or the first days after arriving in a hot climate, as a deliberate reduction period. Cut volume and intensity, extend the timeline, and expect the work to feel harder than the pace suggests.
And recognise the state you are in. The heat index and hydration calculator takes acclimatisation as an input, because the same conditions carry different risk depending on where you are in that fourteen-day window.