What "spacing" actually means in these studies
In this research, spacing means spreading an equivalent amount of practice across separate sessions instead of massing it into one block. The meta-analysis of spacing and retrieval practice in mathematics defines the spacing effect specifically as the change in retention seen when practice is distributed across sessions rather than crammed together [1]. That definition matters: the comparison is about when practice happens, not how much of it there is.
A second meta-analysis plus experimental work on mathematics learning frames the question the same way, comparing spaced practice with massed practice and measuring how much performance improves [2]. It also asks whether the complexity of the math task changes the size of the spacing benefit, which suggests the answer may not be uniform across all kinds of problems [2].
Does spacing produce larger math gains?
The short answer from these studies is yes, at least for retention. The meta-analytic review reports learning gains favoring distributed over massed practice in mathematics [1], and the second meta-analysis and experimental program likewise reports evidence for the spacing effect in math learning [2]. Because both are syntheses of many studies rather than single experiments, the finding is not tied to one classroom or one age group.
The effect also shows up beyond math drills. In a study of 115 fifth-grade children, those given spaced cognitive training improved on both the training tasks and on transfer tasks, compared with an active control group that received math instruction [3]. That is a different kind of outcome, working-memory training rather than math practice itself, so it should be read as supporting evidence for spacing in learning generally rather than as a direct measure of math achievement.
The benefit may depend on how hard the problems are
One of the open threads is task complexity. The experimental and meta-analytic work explicitly investigates whether the spacing effect changes with the complexity of the mathematics task [2]. In plain terms, easy and hard problems may not reward spacing equally, which would mean the practical advice depends on what a student is practicing. The evidence supports the existence of a spacing benefit in math, but it does not establish that the benefit is the same size for every type of problem [2].
What about mental fatigue?
This is where the evidence runs out. None of these studies measured mental fatigue as an outcome: the meta-analytic review focuses on learning gains and does not address fatigue [1], the task-complexity work likewise measures performance improvements rather than tiredness [2], and the children's training study reports performance on training and transfer tasks without a fatigue measure [3]. So the common intuition that spacing keeps students fresher is plausible, but it is not something these studies tested.
What the evidence does support is narrower and still useful: spreading the same amount of math practice across sessions is associated with better retention than massing it [1][2]. Whether that comes with less mental fatigue is a separate question that would need studies measuring tiredness directly, and on the current evidence it remains unanswered.
Sources used in this answer
A meta-analytic review of the effectiveness of spacing and retrieval practice for mathematics learning
The paper is a meta-analysis examining the effectiveness of spaced and retrieval practice for mathematics learning. It compares distributed versus massed practice and reports learning gains. However, it does not address mental fatigue, which is not part of the study’s focus.
Spacing and task complexity in mathematics learning
The paper examines the spacing effect in mathematics learning through meta-analysis and experimental studies, comparing spaced practice with massed practice and measuring performance improvements. It does not address mental fatigue as an outcome, limiting its relevance to that aspect of the inquiry.
Spaced cognitive training promotes training transfer
The paper investigates the effects of spaced versus massed practice on training and transfer in fifth-grade children, using working memory training and a control group receiving math instruction. It reports performance improvements on training and transfer tasks, but does not address mental fatigue as an outcome.