Listening practice: The clock that tells your brain when to sleep

You may think of sleep as something that simply happens when you are tired. But inside your body, timing is being managed all day. According to the National Institute of General Medical Sciences, circadian rhythms are physical, mental, and behavioral changes that follow roughly a twenty-four-hour cycle. They are found in animals, plants, and even microorganisms. In humans, these rhythms help guide sleep patterns, body temperature, appetite, digestion, and hormone release. The biggest outside signals are light and darkness. In other words, your body is not only asking, am I tired? It is also asking, what time does the outside world seem to be?