Heat buildup is one of the most underestimated risks in summer cycling. Wind from riding speed can hide the real load on your body while core temperature continues to rise.
Avoid peak midday heat, drink before you feel thirsty, and plan shaded rest points into the route.
If you feel dizzy, nauseous, unusually weak, or stop sweating, stop riding immediately, cool down in shade, and replenish fluids and electrolytes.