> What's the term for 24-hour cycles that are part of the human body's internalclock?And the answer: circadian rhythms. Photo credit: kanyanat wongsa/ ShutterstockThe cycle of the sun affects everydaylife for humans, making us feel awake at sunrise and more drowsy after the sunsets. This pattern is part of our biological clock, or circadian rhythms, whichalso affect our moods, feelings of hunger, and mental alertnessWhether it be exam week, working the night shift, or an especially funafterparty, there's one internal system you can thank for the feelings ofweirdness that come the morning after (no, we're not talking about a hangover):circadian rhythm. Our internal time-keeping device is calibrated to the settingof the sun (i.e., the disappearance of natural light), so when we mess with it,things can fall out of wack pretty quickly. Coming from the Latin for "Circadiem" meaning "approximately a day," all living organisms have their owncircadian rhythms that dictate eating, sleeping and mating. The main regulator of circadian rhythm can be found in the hypothalamus, a smallarea at the base of your brain that is responsible for connecting the nervoussystem to the endocrine system. A specialized bundle of cells within thehypothalamus is connected to the optic nerve, thus linking the regulation ofbodily functions with the setting of the sun. In the morning, when the optic nerve senses light, the hypothalamus sendssignals to raise our temperature, heart rate and blood pressure, while delayingthe release of hormones that aid sleep like melatonin. Interestingly, our bodiesrise in energy level until about mid-morning, when mental faculties such asmemory and alertness peak. This peak is generally followed by an afternoon lull– in fact, while our desire to sleep is strongest between 2 AM and 4 AM, a closesecond is 2 PM and 3 PM. While it sounds counterintuitive, midday napping is anatural phenomenon in many living animals, and recent research suggests that itmay even be in our best interest (but please, don't cite this article next timeyou get caught dozing at your desk). Learn more about our bodies' internal rhythm below.