Suppose a student just finishes writing his week-long Examinations, and the vacation follows. Still, even on the next day, without any effort he will wake up at the same time early in the morning! Interesting?! Let us see how it happens.
Various changes occur in the body day and night. These changes are cyclical, and occur periodically, and are called circadian rhythm. Thus, there is a biological clock  in our body. The aim of the technique of "how to wake up early in the morning", is to set an alarm in this biological clock. It has been found that these rhythmic changes are controlled by a centre called suprachiasmatic nuclei in hypothalamus (in the brain).
This centre secretes a hormone called ACTH (Adreno Cortic Trophic Hormone), whose levels in the body fluctuate during day and night, but its maximum and minimum levels occur periodically in a fixed manner in a 24 hour cycle. That is, its level is minimum at midnight (during deep sleep) and maximum just before awakening. When there is an increase in the level of ACTH, it acts on the Adrenal gland, and causes increase in the secretion of epinephrine and norepinephrine (sympathetic discharge). One of the effects of this mass sympathetic discharge, is reinforcement of alert, attentive state necessary for effective action causing awakening.
By rising in the morning at a fixed time for the first seven days, we are allowing the rise in th the level of ACTH to occur early in the morning, exactly at that time. On the 8 day, even if we do not plan to rise at that time, the rise in the level of ACTH occurs early in the morning at that time automatically, and we wake up again at that time.
Hence, if you want to wake up early in the morning at a specific time, manage to wake up at that time for just seven days with the help of an alarm clock. Thereafter, you do not have to bother. You will automatically wake up at that time each morning. This is because in a week 's time, a circadian rhythm sets in your mind, and accordingly the adrenaline activation hormone ACTH gets secreted at that specific time thus waking you up. - Dr. J.N. Reddy