Charlene is 12, and her elderly aunt has come for a visit. The aunt is using the spare bed in Charlene's room. Several times during the night, the aunt awakens briefly, and then falls back to sleep. Charlene doesn't awaken at all during the night. This difference in sleep patterns can best be explained by research that shows

during adulthood the proportion of slow-wave sleep gradually declines

Explanation:

Slow-wave sleep: the term "slow-wave sleep" is also denoted as SWS, and is determined as an individual's phase-three sleep that is considered as the "deepest phase" of NREM (non-rapid eye movement) sleep and is therefore distinguished through "delta waves" which is being measured by EEG. During this phase of sleep, sleepwalking and dreaming can happen and is often considered as an important part of "memory consolidation".

In the question above, the given statement states that the difference in the sleep patterns of Charlene and her elderly aunt can be best explained via research that explains that during the adulthood proportion of an individual's life the "slow-wave sleep" tends to decline gradually.