Published in the medical journal Neurology, researchers found that participants who were more active during a specific window of the day had an increased risk of dementia, a condition characterized by declines in memory, language, reasoning and problem-solving skills. Guided by the brain and influenced by light exposure, circadian rhythm is a roughly 24-hour cycle that determines when we become sleepy and when we’re more alert. In addition to regulating sleep and wake cycles, it also regulates hormones, body temperature and digestion.