It is tiring at times to maintain focus.  Why is this so?  For the mind wants to chatter.  It is like a little child filled with
curiosity who speaks incessantly and wanders everywhere it can in search of new
wonders.  Is it any wonder you find it
difficult to quiet the mind in meditation? 
But even children must rest. 
Sleep ensures this, but when the mind is refreshed, practice training it
not to wander quite so much by contemplating a thing or a phrase.  Do your best to keep the mind focused on this
single thing.  Not only will you be more
peaceful throughout your day, but you will increase your connection with the
greater aspect of the Self which cannot be heard above the chatter.