Someone has asked, "What is the difference between samadhi and sleep?" To put it simply, people who are in samadhi will be sitting in an upright posture with perfectly straight backs, not leaning in any direction. Perhaps their breathing will stop or their pulse will cease, so that they appear to be dead; however, awareness and perception remain. Someone in samadhi can sit for an entire day, for ten days, or even for a month without moving. If a person is asleep, however, his head is usually askew, his body is leaning, and he is not in control of himself. He still breathes, and his breathing may be as noisy as thunder. These are the differences.
The process of Chan meditation is just like that of studying. One proceeds from elementary school to high school, then to college, and then on to graduate school, passing through four stages before one can earn a Ph.D. Likewise, the Dharma-door of Chan meditation is also divided into steps, the Four Stages of Dhyana-concentration, which are briefly explained below:
The First Dhyana is called the Stage of the Happiness Leaving Birth. It takes you beyond what living beings can experience, into a state of happiness beyond what ordinary living beings know. This happiness is found in the spiritual skill of our inherent nature. When you reach the samadhi of the First Dhyana, your breath ceases. Your outer breathing stops and your inner breathing begins to function, just as if you'd entered a state of winter hibernation. Your mind at this time is as pure as water and as clear as a mirror. You can know your fundamental identity and can be aware that you are sitting and meditating.
The Second Dhyana is called the Stage of the Happiness of Producing Samadhi. In a state of concentration, an incomparable happiness comes forth. We refer to it as "taking the bliss of Dhyana as food and being filled with the joy of Dharma." Happiness such as this leaves one unaware of hunger. Thus you can sit for many days without any problem. However, you must not become attached to this state, for as soon as you become attached, all your previous efforts will be wasted and you will enter a demonic state. So you must be extremely careful. At the stage of Second Dhyana, not only does the breath stop, but the pulse stops as well. When one leaves concentration, these processes resume their normal functioning.
The Third Dhyana is known as the Stage of the Wonderful Bliss of Leaving Happiness. One reaches a wonderful, inexpressible happiness that leaves behind the happiness found in the Second Dhyana. You feel that absolutely everything is part of the Buddhadharma, and everything is a source of joy. In this stage, not only do the breath and the pulse stop, but the thinking processes also cease. At this point, you no longer think of good or evil, or right or wrong; not even a single thought arises. All the same, you mustn't feel that such a state is extraordinary. It's simply one step along the way, and it's a million miles away from putting an end to birth and death.
The Fourth Dhyana is known as the Stage of the Purity of Renouncing Thought. At this stage, not even the thought of happiness remains. You've already done away with it, and you've come to a state of purity in which there's no further action. This is the stage where "There is nothing to do, yet nothing is left undone." The attainment of the Fourth Dhyana is merely necessary part of the process of developing skill in meditation. There is nothing extraordinary about it. Do not mistakenly think that you have already realized the fruition. If you have that idea, you would be making the same mistake as the Unlearned Bhikshu, and you could fall into hells.
One who reaches the level of the Fourth Dhyana is still considered an ordinary person. If he continues to make vigorous progress and realizes the state of the Five Heavens of No Return, only then has he realized sagehood. Even so, he still hasn't put an end to birth and death. One must transcend the Triple Realm before one can end birth and death. This point must be made clear: different stages should not be mixed up.