Sleep stages are defined in terms of the a. amount of time spent sleeping b. length of restful sleep experienced c. occurrence of brainwave patterns d. number of dreams experienced

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There are five stages of sleep -- the first four are NREM (non rapid eye movement) the last is REM (rapid eye movement). During NREM the muscles retain the ability to function and there is very little muscle activity. Our eyes do not move in NREM sleep, however, in REM sleep our eyes dart back and forth (which is the 'rapid eye movement') while the rest of our body's muscles are completely paralyzed.
In the stages of NREM sleep, the first is very light sleep and the fourth is very deep sleep. The deeper the sleep, the stronger the brainwaves, so it's C, the occurrence of brain wave patterns.

I best believe it is C. occurrence of brainwave patterns.