The Blessed Buddha once said:
Bhikkhus, just as this body, is sustained by feeding, exists in dependence on feeding
and cannot survive without food, exactly & even so are the Seven Links to Awakening
also sustained by feeding, they can also only exist in dependence on feeding and they
cannot survive without feeding... And what, bhikkhus, is the feeding of the emergence
of any yet unarisen Tranquillity Link to Awakening and also feeding of the completion
by cooling of already arisen Tranquillity Link to Awakening? There are 2 kinds of Calm:

Tranquillity of the Body (kāya-passaddhi) & Tranquillity of the Mind (citta-passaddhi)!
Frequently giving careful and rational attention to them, is feeding the arising of any
unarisen Tranquillity Link to Awakening and also feeding of the gradual fulfilment of any
already arisen Tranquillity Link to Awakening... And what, Bhikkhus, is the starving that
obstructs all emergence of a yet unarisen Tranquillity Link to Awakening & which also
hinders any already arisen Tranquillity Link from reaching fulfilment by development?
There are these two kinds of serene & composed calm, which should be differentiated:

Tranquillity of the Body (kāya-passaddhi) & Tranquillity of the Mind (citta-passaddhi)
Not giving frequent, careful and rational attention to them; not considering them much
and often; is the starving that prevents an unarisen Tranquillity Link to Awakening from
arising and also blocks any already arisen Tranquillity Link to Awakening from reaching
any complete fulfilment by mental training and progressive development by meditation...

Comments from the classical commentaries:
Peace is the characteristic of the Tranquillity Link to Awakening (Passaddhi-sambojjhanga).
Stilling of body, feeling, perception, any mental construction and the changing consciousness
is the purpose of the quality of Tranquillity. Settled, serene and stilled calm (samatha) is the
excessively pleasant manifestation of the Tranquillity Link to Awakening. A smiling mountain!
When the mirror moves one cannot see anything clearly in it. So also with mind: When stressed
& agitated, mind cannot figure out what is long-term good. But when stilled and imperturbable
mind can cut right through any distraction and attain absolute certainty and understanding...

Further conditions helpful for the emergence of the Tranquillity Link to Awakening are:

1: Eating good and fine food...
2: Living in a pleasant climate...
3: Maintaining a comfortable posture without pain or distress...
4: Staying ballanced in all situations and regarding all aspects...
5: Avoidance of restless, anxious, agitated and stressed people...
6: Friendship with bodily and mentally calmed people, who meditates much!
7: Commitment to calm down the mind by cultivating quiet and tranquillity!

There is Tranquillity of the Body and there is Tranquillity of the Mind! These mutually
depend upon and enhance each other into even deeper stilling. Awakening is solid calm!

Sources (edited extracts):
The Grouped Sayings of the Buddha. Samyutta Nikaya.
Book [V: 65-6+102-8] 46: Links. 2+51: Group & Nutriments....