The difference between dream and sleep practice somewhat parallels the difference in the practice of calm abiding (zhiné) when an object is used and when there is no object. Similarly, in tantric practice, dream yoga is used to generate the divine body of the meditational deity (yidam), which is still in the realm of subject and object, while sleep yoga develops the mind of the deity, which is pure non-dual awareness. In one sense, dream practice is a secondary practice in Dzogchen because it is still working with vision and images, while in the sleep practice there is neither subject nor object but only non-dual rigpa.
When the student is introduced to Dzogchen practice, practices with attributes are usually taught first. Only after some development of stability is practice without attributes begun. This is because the dominant style of our consciousness has to do with attributes, with objects of the subject with which one is identified. Because we are constantly identified with the activity of the moving mind, in the beginning our practice must provide something for the mind to grasp. If we are told, “Just be space,” the moving mind cannot make sense of it, because there is nothing to hold. It tries to make an image of emptiness in order to identity with it, which is not the practice. But if we say that something is to be visualized and then dissolved and so on, the moving mind feels comfortable, because there is something to think. We use the conceptual mind and objects of awareness to lead the mind to awareness without attributes, which is where the practice must go.
For example, we are told to imagine the body dissolving—that sounds fine, it can be pictured. After the dissolution, there is a moment in which there is nothing to grasp, and this provides the situation in which the prepared practitioner can recognize rigpa. It is similar to counting down from ten—ten, nine, eight—until zero is reached. There is nothing to grasp in zero, it is the tigle of empty space, but the movement leads us there. Counting down to emptiness is similar to using practice with attributes to lead us to the emptiness of practice without attributes.
Sleep practice actually has no form, so there is nothing on which to focus. The practice and the goal are the same: to abide in the inseparable unity of clarity and emptiness, beyond dualistic separation of perceiver and perceived. There are no qualities, no up or down, no inside or outside, no top or bottom, no time or boundaries. There are no distinctions at all. Because there is no object for the mind to grasp as there is in dream, sleep yoga is considered more difficult than dream yoga. Becoming lucid in a dream means the dream is recognized; it is the object of awareness. But in sleep practice, the recognition is not of an object by a subject but is the non-dual recognition of pure awareness, the clear light, by awareness itself. The sensory consciousness is not functioning, so the mind that relies on sensory experience is not functioning. The clear light is like seeing without an eye, an object, or a seer.
This is analogous to what occurs in death: it is harder to become liberated in the first bardo, the primordially pure (ka-dag) bardo, than in the subsequent bardo, the clear light (od-sal) bardo, in which images arise. At the time of death, there is a moment of total dissolution of subjective experience into the base prior to the appearances of the bardo visions. In that moment, there is no subjective self, just as daily experience ends in the dissolution of sleep. We are gone. And then dreams arise in sleep, or images arise in the bardo, and as they are perceived the force of karmic tendencies creates the sense of a perceiving self experiencing the objects of perception. Caught up again in dualism, we continue in samsaric dream if asleep or continue toward rebirth if in the bardo.
If we accomplish sleep practice, we can become liberated in the primordially pure bardo. If we have not accomplished sleep yoga, we will encounter the subsequent bardo visions, during which, if accomplished in dream practice, we are more likely to become liberated. If we have not accomplished either dream or sleep practice, we continue to wander in samsara.
You must decide for yourself which of these practices is most suitable. Dzogchen teachings always stress the importance of knowing yourself, recognizing your capacities and obstacles, and using that knowledge to practice in the manner that will be most beneficial. That said, there are only a few people for whom sleep practice will be easier than dream practice, so I generally recommend beginning with dream practice. If your mind is still grasping, it makes sense to begin with dream yoga, in which the mind can fasten on the dream itself. After stability in rigpa is developed, sleep practice may be easier to accomplish because there is a strong experience of not grasping, of not being a subject, which is the situation in sleep. Another reason I recommend starting with dream yoga is that it usually takes much longer for the practitioner to become lucid in sleep than in dream. Practicing for a long time without apparent results may result in discouragement, which can become an obstruction on the path. Once you have some experience in either of the yogas it is good to continue and reinforce the practice.
The two yogas ultimately lead into one another. When dream practice is fully accomplished, the non-dual awareness of rigpa will-manifest in dream. This leads to many dreams of clarity and finally to the dissolution of dreams into the clear light. This is also the fruit of sleep practice. Conversely, when progress is made in sleep yoga, dreams will naturally become lucid and dreams of clarity will spontaneously arise. The lucid dreams can then be used for the development of the flexibility of mind as previously described. Final success in either practice requires that the pure presence of rigpa be recognized and stabilized during the day.