If your pillows don't pass this test then it's time to throw them away.
"Pillows are an important area of the sleep sanctuary," says Rebecca S. Robbins, a sleep researcher at Cornell University.
"Our pillows, just like our mattresses, accumulate dust mites."
Therefore, it’s important to replace your pillows frequently, about every two or three years.
Robbins test for telling if pillows have lived their life to the fullest:
"Fold your pillow in half and if it doesn’t spring back immediately you have a dead pillow."
Source: House Beautiful