A section of the brain called the hypothalamus controls the circadian rhythm of animals. At night the brain releases melatonin, which makes you feel tired. Although the circadian rhythm is built-in and can work without exposure to daylight, it can be altered by outside conditions. If you have ever experienced jet-lag, you know what it feels like when the circadian rhythm tries to acclimatize to new conditions. Travel affects the circadian rhythm because darkness encourages the brain to release the sleep hormone melatonin, while daylight releases cortisol to wake the brain.

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