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 Sailing Alone Around the Room: New and Selected Poems, by Billy Collins
At the first chink of sunrise, the windows on one side of the house are frosted with stark orange light,
and in every pale blue window on the other side a full moon hangs, a round, white blaze.
I look out one side, then the other, moving from room to room as if between countries or parts of my life.
Then I stop and stand in the middle, extend both arms like Leonardo's man, naked in a perfect circle.
And when I begin to turn slowly I can feel the whole house turning with me, rotating free of the earth.
The sun and moon in all the windows move, too, with the tips of my fingers, the solar system turning by degrees
with me, morning's egomaniac, turning on the hallway carpet in my slippers, taking the cold orange, blue, and white
for a quiet, unhurried spin, all wheel and compass, axis and reel, as wide awake as I will ever be.