each of these subsystems update what they are reacting to at a different rate. Your visual cortex can cohere in less than half a second. A stress hormone like cortisol, on the other hand, has a half-life of 60–90 minutes and so can take up to 6 hours to fully clear out after the onset of an acute stressor. This means that if we switch what we pay attention to more often than, say, every 30 minutes, our system will be more or less decohered—different parts will be “attending to” different aspects of reality.2 There will be “attention residue” floating around in our system—leftovers from earlier things we paid attention to (thoughts looping, feelings circling below consciousness, etc.), which crowd out the thing we have in front of us right now, making it less vivid.

Inversely, the longer we are able to sustain the attention without resolving it and without losing interest, the more time the different systems of the body have to synchronize with each other, and the deeper the experience gets.

FROM:

Henrik Karlsson

Almost Anything You Give Sustained Attention to Will Begin to Loop on Itself and Bloom

Source

This is part of what Nadia talks about in how to focus on one thing for the jhanas and meditation in general.