According to Laundry Experts, You Should Be Washing Your Sheets at Least Once a Week

you don’t wake up feeling sweaty, small amounts of perspiration can transfer to your sheets and pillowcases.

Warm bedrooms, heavy blankets, and humid weather can increase moisture buildup.
Natural body oils

Your skin produces oils that help keep it moisturized. These oils can transfer to pillowcases and sheets, especially around areas that touch the bedding for several hours.

This is one reason pillowcases can develop discoloration over time, even when they appear clean at first.
Dead skin cells

Your skin naturally sheds dead cells throughout the day and night. Some of these cells end up on your mattress, sheets, and