I really need to get some sleep.
Sleep is a naturally recurring state of mind and body
It's really hard to define sleep but that is a solid one.
It is characterized by altered consciousness
Ohh yeah, unless you're lucid dreaming though. 
It is distinguished from wakefulness 
That's better, since you can be conscious while sleeping.
Sleep
Sleep is a naturally recurring state of mind and body, characterized by altered consciousness, relatively inhibited sensory activity, reduced muscle activity and inhibition of nearly all voluntary muscles during rapid eye movement (REM) sleep, and reduced interactions with surroundings. It is distinguished from wakefulness by a decreased ability to react to stimuli, but more reactive than coma or disorders of consciousness, sleep displaying very different and active brain patterns.
Sleep occurs in repeating periods, in which the body alternates between two distinct modes: REM sleep
Sleep yoga") can remain conscious while in delta-sleep.
Delta wave activity has also been purported to aid in the formation of declarative and explicit memory formation.
Cultural and religious role.
In Advaita Vedanta, deep dreamless sleep coexists with wakefulness and dreaming in turiya, considered the background of the higher state of consciousness. If one can stay aware or conscious while in deepest dreamless sleep, a deep meditative state (known as "jagrat sushupti") is said to be achievable.
This notion of