A circadian rhythm is a roughly 24-hour cycle (reflecting the amount of time it takes for the Earth to complete a rotation), in the biochemical, physiological or behavioural processes of most life on Earth, including humans. The term “circadian” comes from the Latin circa, “around”, and diem or dies, “day”, meaning literally “approximately one day”. The study of circadian rhythms and the internal clocks is known as Chronobiology
These circadian rhythms are controlled by a ‘master clock’ in our brain called the suprachiasmatic nucleus. It uses signals like light and darkness to know when to release certain hormones and neurotransmitters that tell us when to wake up and be active or to withdraw and go to sleep. From the beginning of time, people have awakened to morning light and fallen asleep in evening darkness. That’s how we’re genetically programmed to operate: We’re energized during the day and sleepy at night.
This is how it works:
As dawn approaches, our body clock responds to the light by producing the hormones cortisol, adrenalin, and serotonin. Cortisol, produced by the adrenal glands, wakes us up and gets us going in the morning. Serotonin helps us regain consciousness again, and adrenalin keeps us energized and active. As the morning wears on, the body clock signals cortisol levels to drop, while continuing to increase levels of adrenalin and serotonin.
As the day progresses, our body temperature, which starts out low, begins to rise, and so does our metabolism (the trillions of intricate biochemical processes that keep us alive). The body clock signals us to become hungry, and it signals the liver and digestive system to process nutrients. By midday, our metabolism is reaching its peak. That’s why midday is the right time to have your largest meal, because midday is when your body is primed to deal with a lot of food.
Later in the afternoon, the body clock reduces its output of active, energetic hormones. Body temperature begins to fall, metabolism slows down, and we begin to wind down. Because of these slowdowns, we are poorly prepared to digest a large dinner when most Americans have dinner, at around 6:00 to 9:00 PM, which is why smaller meals at dinner time are better suited to your inner rhythm. As evening progresses and light fades, the body clock signals the pineal gland to convert serotonin into melatonin and we become lethargic. As melatonin and other sleep hormones increase, our temperature continues to drop, and we start thinking about withdrawing for the night; it becomes difficult to stay awake. This is the best time to fall asleep. Body temperature continues to drop as melatonin is released into the bloodstream.
Melatonin continues to be released until the body clock perceives a gradual increase of light. As dawn approaches, the production of melatonin shuts down and the body clock begins the active cycle again, releasing cortisol. As sunlight increases, the body clock begins producing adrenalin and serotonin. And another day begins.