What's the term for 24-hour cycles that are part of the human body's internal clock?
And the answer: circadian rhythms.
The cycle of the sun affects everyday life for humans, making us feel awake at sunrise and more drowsy after the sun sets. This pattern is part of our biological clock, or circadian rhythms, which also affect our moods, feelings of hunger, and mental alertness
Whether it be exam week, working the night shift, or an especially fun afterparty, there's one internal system you can thank for the feelings of weirdness that come the morning after (no, we're not talking about a hangover): circadian rhythm. Our internal time-keeping device is calibrated to the setting of 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 "Circa diem" meaning "approximately a day," all living organisms have their own circadian rhythms that dictate eating, sleeping and mating.
The main regulator of circadian rhythm can be found in the hypothalamus, a small area at the base of your brain that is responsible for connecting the nervous system to the endocrine system. A specialized bundle of cells within the hypothalamus is connected to the optic nerve, thus linking the regulation of bodily functions with the setting of the sun.
In the morning, when the optic nerve senses light, the hypothalamus sends signals to raise our temperature, heart rate and blood pressure, while delaying the release of hormones that aid sleep like melatonin. Interestingly, our bodies rise in energy level until about mid-morning, when mental faculties such as memory 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 close second is 2 PM and 3 PM. While it sounds counterintuitive, midday napping is a natural phenomenon in many living animals, and recent research suggests that it may even be in our best interest (but please, don't cite this article next time you get caught dozing at your desk).
Learn more about our bodies' internal rhythm below.