Heat syncope is a fainting (syncope) episode or dizziness that usually occurs with prolonged standing or sudden rising from a sitting or lying position.

Factors that may contribute to heat syncope include dehydration and lack of acclimatization.

Symptoms of heat syncope include:
• Fainting (short duration)
• Dizziness
• Light-headedness during prolonged standing or suddenly rising from a sitting or lying position

First Aid
Workers with heat syncope should:
• Sit or lie down in a cool place.
• Slowly drink water, clear juice, or a sports drink.

From: cdc,gov