Two separate claims are folded into this question, and they have very different amounts of support behind them. One is that a suggestion given under hypnosis can shape what a person dreams about. The other is that such suggestions, repeated over weeks or months, can lengthen the REM portion of the sleep cycle. The first has a thin but real research history. The second is largely speculative.
On dream content, the older literature is genuinely interesting. Researchers going back decades compared dreams that followed a post-hypnotic instruction to dream about a named topic, and found that suggested themes did sometimes surface, both in hypnotic states and in later natural sleep. People who score high on hypnotic suggestibility also tend to report more vivid imagery and better dream recall, which fits the idea that a directed suggestion could nudge the emotional tone or subject of a dream. None of this means a dream can be scripted. It means the content can be biased toward a theme more often than chance would predict.
REM-cycle duration is where the question outruns the evidence. The cleaner sleep findings on hypnosis concern slow-wave sleep, not REM. In one controlled study, healthy young women who listened to a suggestion to “sleep deeper” before a nap showed a marked increase in slow-wave sleep measured by EEG, an effect concentrated in those high in suggestibility. That is a deep, dreamless stage, and the result has not been shown to transfer to REM length, nor to hold up across extended periods of nightly use. Claims that hypnotic reinforcement durably extends REM over time are not established.
A few distinctions worth keeping straight:
- influencing the theme of a dream is plausible and has some support
- increasing slow-wave sleep with suggestion has at least one solid controlled result
- durably changing REM-cycle duration over weeks is not a demonstrated effect
There is also a practical caution. Heavily scripting dream content, even if it worked, would not obviously be desirable, since dreams seem to serve emotional processing that may depend on a degree of spontaneity. Clinicians who use imagery rehearsal for recurrent nightmares, for instance, rework a single distressing dream rather than trying to engineer the whole night.
The honest summary is uneven. A post-hypnotic suggestion can, in some people, tilt what a dream is about. Whether it can stretch the REM cycle over the long term is a question the research has not answered, and any guide that states otherwise is reporting a hope rather than a finding.