"The Pains of Sleep" begins with a bedtime routine that doesn't sound painful at all: the speaker describes the way he usually says his prayers before he goes to sleep.
Or, rather, he describes how he doesn't pray. "It hath not been my use," he says, to pray "with moving lips or bended knees." In other words, it's not his custom to get down on his knees or say a formal prayer, as many people would.
Instead, he performs a quiet little ritual of his own. Listen to his sibilance in these lines:
[...] silently, by slow degrees,
My spirit I to Love compose,
The mixture of hushed /s/ and /z/ sounds here suggests both his outer silence—no "moving lips" here—and his inner stillness. All he has to do to pray, he says, is to "compose" himself, to quiet his "spirit" down. Noiselessly, gradually, he orients his soul toward "Love." The capital "L" there suggests that this speaker is thinking of Love, not as an abstract principle, but as a name for God.
In these lines, the speaker presents himself as a man with a deep, quiet, and personal faith in a loving deity. He doesn't need to make any special gestures or mouth any particular words to feel as if God is right there with him; all he has to do is open his mind to the spirit of "Love."
But the poem's title, with its "Pains," foreshadows a change: all will not remain well for this faithful speaker. His trust in God, he'll discover, can't save him from suffering. Coleridge knew this all too well: this autobiographical poem will reflect on his own experiences.