Why would a shaky memory benefit more from a sleep cue?
The intuition is that a cue during sleep works like a nudge to replay a memory. If a memory is already solid, the nudge adds little; if it is fragile, the nudge may be what tips it into lasting storage. In one study, researchers manipulated how strongly pairs of items were encoded before sleep and then cued some of them during sleep. Sleep cues enhanced memory only for the weakly encoded pairs [1]. That is consistent with the idea that cueing has more room to help when the memory is not yet secure.
A second study points the same way using a different measure: how accurate the memory was before sleep. Targeted memory reactivation — playing sounds or odors tied to specific learned material during sleep — produced larger benefits for low-accuracy memories than for high-accuracy ones [2]. In other words, the cue helped most where the pre-sleep memory was least reliable. Both results fit a "room to improve" picture rather than a "moderate sweet spot" picture.
What would it take to show a moderate sweet spot?
A true moderate-is-best claim needs three groups compared head-to-head: weak, moderate, and strong pre-sleep memory, all given the same sleep cue. Neither of the cueing studies above set up that three-way comparison. The first compared weakly and strongly encoded pairs [1]; the second compared low- and high-accuracy memories [2]. A pattern where the weak group gains most is not the same as a peak in the middle, and the evidence so far does not distinguish those two shapes.
There is also a related finding that complicates any simple "strong memories need no help" story. In a separate line of work, when people learned unfamiliar material with low prior knowledge, adding extra pre-sleep training opportunities restored sleep-associated memory benefits [3]. That study did not use sleep cueing, so it speaks to pre-sleep learning strength rather than cueing specifically, but it suggests that boosting a weak memory before sleep can change what sleep does for it.
Does this change how you would use sleep cueing?
If you are trying to use sleep cues to shore up something you have just learned, the evidence leans toward cueing material you are still shaky on rather than material you already know cold. The cueing studies found the biggest gains where pre-sleep memory was weak or inaccurate [1][2]. That is a practical hint, not a guarantee: the studies measured memory in laboratory learning tasks, and the sizes of the effects are not something the abstracts let us compare across everyday situations.
The honest summary is that the "moderate rather than weak or strong" framing is not what these studies tested. What they support is a gradient — weaker or less accurate memories tend to gain more from a sleep cue than stronger ones [1][2] — while the question of whether a middle level would outperform both ends remains open.
Sources used in this answer
Sleep reactivation did not boost suppression-induced forgetting
Sleep-based memory reactivation enhanced memory only for weakly encoded pairs, not strongly encoded ones, in a study of suppression-induced forgetting.
The benefits of targeted memory reactivation for consolidation in sleep are contingent on memory accuracy and direct cue-memory associations
Targeted memory reactivation during sleep produced larger benefits for low-accuracy pre-sleep memories than for high-accuracy ones.
Is prior knowledge essential? Additional training opportunities restore sleep‐associated memory benefits under conditions of low prior knowledge
Additional pre-sleep training opportunities restored sleep-associated memory benefits when prior knowledge was low, though this study did not use sleep cueing.