Some things in life aren’t certain, but one thing is sure: the sun will rise every morning and will set at night. The Earth has turned in a steady 24-hour rhythm for the past 4,5 billion years. Most life forms have adjusted to the day-nigh-rhythm, and to do so, they have evolved an inner clock to handle the changes in light during the day and night and to predict and guide their vital functions accordingly. For example, some plants can predict the sunrise, and they can open their leaves just in time to get the maximum benefit of the sunlight.
In humans, the inner clock (or circadian rhythm) optimizes bodily functions, such as temperature, metabolism, and hormonal functions, based on the time of the day. When working correctly, your inner clock makes you sleepy when it’s time to go to bed in the nighttime and make you feel more alert during the days. Melatonin (“the hormone of darkness”) is the hormone, especially in charge of the timing of falling asleep, and circadian rhythm is heavily controlling its secretion. Melatonin starts to secrete in the nighttime when the day gets darker, and its message to the body is clear: it is night, time to go to sleep. The amount of melatonin decreases during the night, and the secretion stops entirely in the morning before it’s time to get up.
The 24-hour cycle is determined by genetics. However, it needs outside help to stay in check, and the most important external factor is light. The human body works in its best when the inner clock is in sync with the outside environment. That’s why it is highly essential to, for example, get as much light as possible to your mornings, and to avoid artificial lighting and dim the lights before bedtime.
Example Habits to try:
Borbély, A. A., Daan, S., Wirz-Justice, A., & Deboer, T. (2016). The two-process model of sleep regulation: A reappraisal. Journal of Sleep Research, 25 (2), 131-143.
Panda, S. (2018). The Circadian Code: Lose Weight, Supercharge Your Energy, and Transform Your Health from Morning to Midnight. Rodale Books.