Humulin N (insulin isophane) is a man-made form of a hormone that is produced in the body. Insulin is a hormone that works by lowering levels of glucose (sugar) in the blood. Insulin isophane is an intermediate-acting insulin that starts to work within 2 to 4 hours after injection, peaks in 4 to 12 hours, and keeps working for 12 to 18 hours. Humulin N is used to improve blood sugar control in adults and children with diabetes mellitus. Humulin N may also be used for purposes not listed in this medication guide.