it is actually physical energy that is used in mental work. mentally hard things take more energy, the active cells actually get depleted, mental fatigue occurs. the only way around the physical limitations is to properly employ the passage of time so that the required repletions can occur.
the beacon for example. it revealed itself to me a few days ago. place the attention at a given physical spot, draw an imaginary line to the visual vertex (the 2 cones meet at that point back in the middle of the brain). that line can sit there and can be noticed as the body position changes. lots of energy needed to keep that locator process going. mind gets fatigued, slips back into the lazy state.
the beacon thing is continuing to be interesting. i do not yet find it possible to maintain the line for more than a few seconds. i draw the line to my visual vertex, admire it for a few seconds while standing still, start to move & quickly forget about it, always remembering later & having to reestablish it, gets pretty approximate if i'm far away, idle poky exercises of trying to figure out where it is while i'm running or driving, somewhere behind me at some angle when i'm heading out.
generally, the ability to establish points of reference so that one can attempt to refer to something as one continues to explore reality as it rushes at us. maybe i don't know where i am, but i know where something is. that spot over there. so if i know it's there, i'm here, i have that relation, oh, good, i know something. i can relax, try to figure out what's going on.