1Your body gets up in the morning and your mind often does not. Half a day can go by in a sort of waking dream, and whatever meets you in it gets the sleepwalker's answer.
So make waking the moment you decide something, before the day has handed you anything to react to. Thirty seconds on the edge of the bed will do: today I will notice once before I answer. Hanging it on something that already happens every morning means you never have to remember to find a time for it.
Islamic evidence
Let them pardon and forgive. Do you not wish that God should forgive you? (Quran 24:22). Deciding to overlook things is far easier at the start of a day than in the middle of one, while nobody has done anything yet.
“Those who have been graced with bounty and plenty should not swear that they will [no longer] give to kinsmen, the poor, those who emigrated in God’s way: let them pardon and forgive. Do you not wish that God should forgive you? God is most forgiving and…”
Qur'an 24:22
Psychological evidence
In a survey of 286 adults aged 18 to 88, older adults reported letting anger out less and dwelling on emotional events less, and those differences related to their well being, which suggests these patterns are not fixed for life. What it cannot tell you is whether a morning intention shifts them, since nobody tested that. It is offered as a reason to think the settings move at all.
Phillips LH, Phillips LH, Henry JD, Hosie JA, Milne AB. (2006). Aging & mental health · doi
Review detail
Why it works. A moment that comes round on its own is the easiest place to hang a new intention.