Every human brain has a certain capacity of intake, after which it forces the human body to sleep in order to restore itself. This holds good in nearly all cases where human brain is vigorously used.
Quran memorization is one field where human brain memory is used to its full capacity and hence requires rest to restore itself. Moreover, if one does not give rest to it, it either automatically switches off the human body and put it to sleep or stops taking any more information into it. This is precisely why every child and adult who is memorizing Quran, must get adequate amount of sleep and rest.
Forcing a child to memorize Quran by sitting for long hours is not fruitful, instead it can backfire and make the child forget what he has already memorized. So avoid such forceful approach and try to give proper breaks and rest to the child before getting back to the memorization of Quran.
It is recommended that the child gets some rest before going to Qur’an class, even if it is for a short amount of time (e.g. half an hour). This rest significantly helps the child by making him energetic and attentive in class, which will in turn increase his performance in memorizing and perfecting his assignments.
As for students who do not receive any rest before class, they may very well leave the class without any significant gain. The same case is with the adults who are attending Quran study circles or any other Islamic study circles.
A good amount of physical and mental rest assures improved performance in both children and adults. More over, we tend to enjoy Quran memorization if we are doing so with a fresh mind and mood.