For those who like a balance between two extremes, a medium-weight duvet insert is the perfect choice.  Medium Weight Duvet Insert provides a moderate level of warmth, making it suitable for year-round use in many climates.  The mid-weight down comforter core provides enough warmth to keep you cozy without feeling too heavy or light.

However, flannel sheets can be made of cotton, wool, or synthetic fibers, leaving brushed cotton a  good balance of breathable and warm.