in my esv study Bible, it says of this verse:

"my sitting down and rising up is an old testament expression depicting the routines of life.  whether a person is at rest or work, God sees and knows ... that "You understand my thoughts" positively overwhelmed david. God cares about every detail of every person's life.

"the message" version of the Bible says "i'm an open book to you".

these verses continue with the theme of the entire passage that we are intimately known by God, the creator of the universe. 

i posted this word art from my personal devotions earlier this morning and it fits so amazingly with this verse to me! if i am perfectly known by the God of the universe, the One who made the moon and the stars, the land and the seas and all of the galaxies, what have i to fear? it is about God from the first to the last ... He created me, He loves me, He knows me and He understands my thoughts ... better than i do myself ... the truth of that is all that i really need!

dear Lord, as i journey through this day, remind me that nothing that comes my way has not passed through Your loving hands first. remind me that You know when i sit and i rise, and You know my thoughts {even before i think them}. if anxiousness or fear threatens, please remind me of these truths from Your word! amen.

thanks for reading today ... remember, you are cherished, loved and adored!