Meditation is a state of mind. Sleep is another statement of mind. During meditation there is alpha state and theta state.
Alpha state is just before sleep. When we are wide awake, our brain is in a beta (and sometimes gamma) state, but as we drift into sleep, it shifts to alpha, and then to delta and theta in deep sleep. The alpha state is a light hypnotic state—relaxed, yet focused and receptive. In this state of consciousness, a person is calm, able to absorb new information and become more resourceful and open to new possibilities. Alpha brainwaves vibrate at between 7 and 13 cycles per second
Theta State is the twilight state, being on the threshold of sleep but not yet asleep. You enter into this state normally as you enter sleep (hypnagogic) and as you exit sleep (hypnopmpic). Theta brainwaves correspond to a state of mind associated with dreams and waking dreams, as well as a deep meditative state. Theta brainwaves are slower than the Beta and Alpha activity we normally have in daily life. They vibrate at between 4 and 7 cycles per second. Theta states of mind are normally responsible for those "Eureka!" moments of well known inventors and scientists. They are highly creative states. During this state, you tap into the creative and intuitive resources of your mind. Passive problem solving and creativity occur in this state. Goal achievement also becomes a much easier and more efficient process.
When we are in normal waking consciousness and doing all forms of action, thinking and problem, we are at Beta state of mind at which the brain waves vibrate at 13-40 Hz.
The above classification has been measured by EEGs – alpha, beta, theta and delta.
During sleep, the drop in energy consumption (metabolic rate and oxygen consumption) varies from 2 -10 % of the awakened state.
During the meditative state, this drop goes as much as 16%.
Sleep can never be a substitute for meditation. The meditative state is an interface or relay station between your conscious mind and subconscious mind.