When measuring nocturnal sleep for a person that has narcolepsy using an EEG a distinctive lack or reduction of slow wave sleep activity is indicated. Slow wave sleep brainwaves are the brainwaves with the highest amplitude. Given a 24 hour day, can I infer that the brain of a narcoleptic general operates at a lower mean amplitude than a “normal” brain?
I would not recommend phrasing it that way.
the brain of a narcoleptic generally operates at a lower mean amplitude than a “normal” brain
is very misleading and imprecise, because it suggests the amplitude is lower overall, which it may not be, whereas
a distinctive lack or reduction of slow wave activity
is precise and clear: it is talking about a specific type of brain activity at a particular time of day.
Additionally, the second statement implies nothing about the rest of the day. It could be that there is an increase in slow oscillations during other parts of the day that offsets the change at night, for example. Additionally, it is not typical to talk about "mean amplitude" over a 24 hour period. There is simply no good reason to rephrase it the way you are suggesting.