Your head is in close contact with your pillow all night. Your head can tend to get hot due to the blood flow through your neck and around your face. Keep in mind that your blood temperature is 37oC or 98.6oF. This is quite warm, and when you sleep at night, this heat is transferred to your bedding – including your pillow. A pillow is smaller than a mattress, so this heat has a much lower volume in which it can dissipate than with a mattress. Your face and head are also in intimate contact with your pillow, making it much easier for these to get overheated.