This weakens the argument. It attacks the author’s assumption that a lack of REM sleep makes REM sleep-deprived individuals cranky, as people deprived of non-REM sleep also become cranky. This weakens the causal relationship between a lack of REM sleep and crankiness.

This does not affect the argument. The nature of the dreams one has in REM sleep does not affect the phenomenon the author seeks to explain, which is that a lack of REM sleep makes people more irritable.

This does not affect the argument. While we know that dreams only occur during REM sleep, we don't know how, or whether, a lack of dreams impacts the stress relief process the author claims occurs during REM sleep.

This does not affect the argument. We don't know how, or whether, remembering the dreams that occur during REM sleep affects the stress relief process the author claims occurs during REM sleep.

This strengthens the argument. It offers more evidence to support the causal relationship the author assumes exists. In other words, it supports the author’s assumption that a lack of REM sleep (as opposed to some other factor) makes REM sleep-deprived individuals cranky.