And what is strange, no one has answered the question posed. Perhaps they did not understand.
I am familiar with this phenomenon and there is an assumption about: in fact, we DO NOT wake up a second before the alarm clock.
I think that the brain is just fooling us and upon waking up with the first sound of the alarm clock we have a feeling, or maybe even a "memory" that we remember being awake during the call.
I am still sticking to this explanation, because others seem less likely:
chance or coincidence is in principle possible, but in my entire life I have more often met wake up a second before the alarm clock than a minute or two, or even five. This is very unlikely.
and, as it seems to me, the body cannot tune waking up to such perfect accuracy. It just seems incredible, but this is the first thought that comes to mind after a call (at least for me personally) - "Wow, our brain is so developed that it can figure out when to wake up to within seconds!".
But then, I return to the first explanation above - maybe our brain is not so developed, but rather, on the contrary, is capable of fooling us? It's not the first thing.
I'll tell you how I feel. It is extremely painful for me to wake up when the alarm rings, I wake up very abruptly and the mood is not good, to put it mildly. Therefore, as written in the first answer, probably my body has already tuned in to the time when it is necessary to get up so that it is on time, not an hour earlier than necessary, and plus everything in such a way as not to hear the hateful alarm clock.
My own example. If in the morning I have an important business, an event, then the alarm clock should not be set at all. It's about the mood of the body. Tuning yourself on a subconscious level that you can't sleep in any way, the body itself will give you a "bell" that it's time to get up!
If a person has set himself an alarm for a certain time and wakes up for quite a long time at his call, then his brain has already managed to adapt to this rhythm and adjust the biological clock of the body for this time. In more detail: in the part of the brain, which is called the intermediate, there is the hypothalamus and the pituitary gland, which together form the hypothalamic-pituitary system, as well as the pineal gland, epithalamus, etc. In these structures, the synthesis of some vital hormones that regulate the activity of the vascular, respiratory , motor systems or other endocrine glands. In addition, the hypothalamic-pituitary system has the ability to perceive and interpret the length of daylight hours and provide a certain photoperiodism, that is, they set the biological clock of falling asleep-awakening (as well as the approximate time of food intake, in many animals, and the time of reproduction, because the lengthening of light day, for example, indicates the arrival of spring, then it's time to look for a partner). If your guideline for sleep is not the sun, but an alarm clock, then these structures will provide an increase in the level of melatonin by the time of falling asleep (if you go to bed at a certain time every day), and by the time of waking up, the level of the hormone cortisol rises, which provides an easy rise.
As for the minutes - the most complex, precisely adjusted system, which is the body, obeying its internal rhythms, cannot exactly correspond to some kind of stopwatches and ensure you wake up with an accuracy of a minute :)
On the other hand, early awakening can also have a psychological cause. Maybe you need to wake up early in the morning because of some important thing, maybe something exciting and important awaits you and you are nervous thinking about it. Maybe for this reason you never managed to fall into a deep sleep that night and you, tormented by thoughts about this important day, try not to oversleep, you say to yourself: you cannot sleep longer, we have an important day, do not oversleep! That is, your cortisol level is already elevated. Probably everyone has this happened when you wake up not a couple of minutes, but a couple of hours before the alarm clock, fall asleep again, wake up another hour, then half an hour later, plunging into a disturbing sleep for a short while and then feeling so broken and lost ...
In short, get enough sleep :)