Why did he decide not to ‘fight for sleep’?

Last night I did not fight for sleep
But lay awake from midnight while the world
Turned its slow features to the moving deep
Of darkness, till I knew that you were furled,

Beloved, in the same dark watch as I.
And sixty degrees of longitude beside
Vanished as though a swan in ecstasy
Had spanned the distance from your sleeping side.

Read to the first full stop.