The circadian rhythm is a natural sleep-wake cycle that is regulated by the brain. It is influenced by light, and when the body's natural light-dark cycle is disrupted, it can lead to problems sleeping. In this case, the student's difficulty staying awake during the day and sleeping through the night suggests that their circadian rhythm is out of sync. This could be due to a number of factors, such as changes in their sleep schedule, exposure to bright light at night, or jet lag.