Rest is many things, but a definition I’ve found to be useful over the years is that rest is the cessation of want. This does not define every aspect of rest, as you can also find, for example, rest in action.
The latter is a bit more challenging, and if you are presently short on rest I recommend that you narrow your focus to the former. To rest in this way, that is, through the cessation of want, you must come to the point where you are willing to place all that you wish to achieve in life on hold for the moment. As strange as this might sound, you must also temporarily suspend your desire to finally get some rest!
In this sense, rest is the calm water which can be found in the wake of thankfulness. Thankfulness is a state of being, not an act of accomplishment. Being thankful stills the noise of the seas and the rushing of the multitude of details of living.
Rest and be thankful. – William Wordsworth