The cleaner, fresher and more natural sheets are, the better they are to sleep on. Because of the amount of perspiration given off during an 8-hour sleep, choose sheets that breathe and absorb moisture. Look for untreated sheets made from organic, natural fibers. In winter, cozy up in organic cotton flannel sheets. In the summer, pure linen is best, or go for less expensive organic all-combed pima or percale cotton sateen sheets. Don't buy polyester sheets because they are made with petroleum-based fibers. Sheets treated with resins (no-iron finishes) should be considered toxic; washing can never eliminate the resins. Also, avoid sleeping on pure silk sheets, labeled "dry-clean only," since dry-cleaning emanates toxic fumes.