Nearly all organisms on earth have a circadian rhythm, an internal time-keeping "device" which is calibrated to the rising and setting of the sun.
The term circadian comes from the Latin root "circa diem" which means approximately a day, and it impacts everything from eating and sleeping to testosterone secretion, bowel movements and more.
Your hypothalamus is the main regulator of your circadian rhythm, a small area in the base of your brain, that connects your nervous system to your endocrine system. Within the hypothalamus lives a small group of nerve cells called the suprachiasmatic nucleus, which connects to your optic nerves, allowing the cells to respond to light and dark.
When your optic nerve senses light, the cells in your hypothalamus sends signals to raise temperature, heart rate, blood pressure and delays the release of melatonin, a hormone that makes you sleepy.
When you're exposed to light in the morning, it helps your circadian rhythm know it's time to start the day, thus waking you up, making you more alert and sharpening your mind. Conversely, when the sun sets and the light fades, your circadian rhythm makes you tired by releasing more melatonin, for example.
Your circadian rhythm is controlled by your genes, so everyone's is a bit different (that's why some people are naturally early birds while others are night owls.)
However, your circadian rhythm can be impacted by outside stimuli, such as light, diet, exercise, etc. so you can train your circadian rhythm to naturally wake up early.
Unfortunately, your circadian rhythm is not in tune with your work schedule, nor does it care that you enjoy binging your favorite Netflix series until 3 AM.
Circadian rhythms are aligned with the sun, aka light, which means you can easily disrupt your circadian rhythm with phones, televisions, computers, tablets, and more.
If you want to wake up naturally without an alarm, you need to adjust your circadian rhythm to wake you up at your desired time every morning.