In regards to the "Dark Night" (
knowledges of fear,misery, and disgust ), mettā is especially useful physiotherapy for the brain-influenced-by-negative-mentation, and changing the brain to a positive and/or neutral base.
Knowledges of suffering can be very powerful in that the mentation/mental images/verbal gatling may seem inescapable, therefore linking breathing-in with ease and comfort sensation is a great way to use something that is constant (breath) to constantly re-train the brain to recall and know ease-comfort-sukha.
Richard Gombrich has noted that "the word mettā is the abstract noun derived from mitto (friend), so that "friendliness" is a possible translation [of mettā] (...)" (p.85, Gombrich, What the Buddha Thought, Equinox, 2009).
Therefore, one can inhale-friendliness-expanding-all-cells-with-pleasant-feeling at any moment including at the outset of sitting. Inhaling-friendliness-expanding-all-cells-with-pleasant-feeling can counter restlessness, aversion, perseverating problems from the day (stress,
five hindrances), and general distractions. It is gentle and the sort of care that one wishes to be given to all newborns and to anyone suffering.
The resulting sukha can be then taken as object (access and absorption concentration), or simply be the go-to place when another practice is breaking up.
This inhale-friendliness-expanding-all-cells is also useful if a person is having difficulty remembering a pleasant, comfortable experience by which to initiate ease/comfort/sukha. Here, inhaling-friendliness into the body creates that pleasure.
[Edit: Breathing can be felt enlivening all of the body. When people report fear, then I think it is great to either focus the breathe at the nose or feel how it effects the shoulders, biceps, rhomboid, intercostals, pelvic basin, thighs, calves, feet...all through the body, however, staying away from the chest area (and staying away can be accomplished not with forced avoidance (which is not unlike forced confrontation), but by simply placing attention gently on another part during the inhalation). ]