To me, sadhana is a daily spiritual practice allowing time and space for an individual to turn inward.
As Yogi Bhajan (of the Kundalini yoga tradition) says, "Sadhana is self-enrichment. It is not something which is done to please somebody or to gain something. Sadhana is a personal process in which you bring out your best."
Sadhana could be taking a walk in nature, doing breath work or yoga asanas on a mat, spending time meditating or chanting, reading and reflecting on a poem, or simply watching the sun rise.
Please accept this post as a possible starting point for your own practice today.
From Anne Morrow Lindbergh's Gift From the Sea:
But I want first of all ... to be at peace with myself. I want a singleness of eye, a purity of intention, a central core to my life that will enable me to carry out these obligations and activities as well as I can. I want, in fact — to borrow from the language of the saints — to live "in grace" as much of the time as possible. I am not using this term in a strictly theological sense. By grace I mean an inner harmony, essentially spiritual, which can be translated into outward harmony.