Thomas Gainsborough is famous for extending the handles of his brushes (up to 6ft!) to prevent himself from straying to near to his portraits, and I once heard that another famous artist, whose name escapes me, reputedly ran through the open doorway into another room to decide where the next stroke was going to go and then ran back again.  These examples may be a bit on the bizarre side, but one artist’s trick you do frequently see is that of wearing reading glasses to paint.  If worn just low enough on the nose, it enables an artist to flick their eyes between a detailed view and a distant view without falling into the trap of stepping forwards to apply a detail and then forgetting to step back again.
I know that some very talented artists sit down.  I’ve seen them too.  But I bet you there was once a time when they stood up, which means that when they come to make that mark that isn’t reachable, something in their subconscious, learned in their past, will make them stand up to do it properly.  Also, if you’re doing a fairly small painting you can get away with sitting down as long as you position your chair a good distance from the easel and do not, under any circumstances, allow yourself to move the chair nearer. 

If, like me, you fall into the category of ‘beggars can’t be choosers’ try to keep your easel as far away from direct window light as possible or, failing that, hang a thin cotton sheet (I use a sarong which is perfect) over the curtain rail in your studio.  This will go some way to tempering the changes in light without cutting it out altogether. 

For particularly dark days some artists use special daylight light bulbs in their studios because they prefer to work in a slightly cooler, bluish light, which is more akin to that of the natural outdoor light in the northern hemisphere.