In the TED Talk "Own your body's data" Talithia Williams makes a case for collecting temperature data on your body to monitor the functioning of your thyroid and ovulation cycles. Many people routinely monitor weight, blood pressure, blood sugar, blood oxygenation, resting heart rate, and other indications of how their body is functioning in response to changes in diet and exercise. People with macular degeneration can monitor their vision with an Amsler grid. People have used a small light source held close to the eye to entoptically monitor the growth of a cataract in the lens.