Some people sweat while they sleep because the temperature of their sleep environment is too warm, or if their duvet is too thick for warmer climates. Some materials can trap heat more than others, such as wool, which is a natural insulator.
A person may also sweat overnight because of health conditions such as hyperhidrosis and anxiety, or if they are going through menopause.
A person can consider using a lightweight duvet made from breathable materials to reduce the risk of sweating overnight.