In this activity (3rd on the page), learners investigate circadian rhythms by examining how well people do with estimating time. In order to explore the question, "Do people have built in stopwatches in their heads?", learners work in pairs to conduct two time estimation tests. Learners can also explore other factors that may influence one's ability to estimate time such as age, use of "timing" strategies, time of day, and body temperature. This activity guide also includes a link to a online time estimation experiment, where learners can test their time estimation accuracy.