"Sweet unrest" are the two words that Keats uses to capture the feeling he has when he lies pressed against his lover in the moonlight. Even though there is an oxymoron in the juxtaposition of the two words, the "unrest" is "sweet" because it allows the speaker to savour the moment of love as he lies with his beloved. In these last few lines, the speaker expresses his desire to enjoy such moments for all eternity, but ironically, the speaker realises that the star is essentially different from himself, as the star is remote and distant and unable to express feelings. To be human is to accept both the positives of being able to feel love deeply but also the negatives of recognising that such moments are temporary and that time will pass. Thus this leads the speaker to realise that remaining in "sweet unrest" for all eternity is a hopeless dream. The best he can hope for is to "swoon to death" at the moment of his highest happiness:
Awake for ever in a sweet unrest,
Still, still to hear her tender-taken breath,
And so live ever--or else swoon to death.
The phrase "sweet unrest" therefore appears to be contradictory, because it is difficult to imagine not being able to get to sleep as being "sweet." However, Keats manages to weld into his poem a number of different contradictions, of which this is one, that allow him to explore the transitory nature of human experience and emotion.