What happens inside the brain when we concentrate (meditate), and why does a person find it difficult to continuously concentrate for a long time? That is, why does one need a break after working or studying for a given time, or why do we lose our concentration after a certain time?
The Neurons become refractory which means that they can't fire again until time has passed. Look up Action Potential and the limits of firing rating. There is also a competition for different circuits to become conscious so when one becomes tried the other takes over. This is why it is hard to continue to think about one thing, without soon considering something else.