Until hypnagogia.

To a listener of the story not directly experiencing it himself, he would insist that what's claimed is explicitly impossible, because targeting one's sleep from another dwelling is beyond the means or desires of an average person, and that what is likely being experienced is no more worse than random noises and the type of living generally found in low rent apartments. Furthermore, the listener would declare that the victim is merely oversensitive and weak, and that the victim is experiencing nothing more than the type of frustration one has when trying to get good sleep. And that specifically believing one is being persecuted or attacked like this is very likely sign of intense mental illness.

Meanwhile, for the one actually experiencing it, should they ever catch the attack on recording, they would realize that every bit of genuine deep sleep is specifically targeted. And, failure to achieve deep sleep, ever, will result in horrendous side effects of severe sleep deprivation; far, far worse than what has been imagined by the hearers of such a tale. In that sense the form of attack is perfect. At least up to the point where such means, likely involving AI, becomes commonplace enough for many others to see how it could be done. Because we move toward the perfect surveillance state as technology improves.

Headache, increasing obesity, and eventual heart failure, the before-outlined clearing of the brain's synapses, etc, causing memory loss, sleepiness at first, giving way to insomnia oddly enough, ultimately causing feeble mindedness and an ever dwindling amount of credibility, are what to look forward to if you never can sleep beyond hypnagogia.