For Sirius, it takes a few hours for the rush of adrenaline, the kick that comes from a successful mission to wear off and for his feet to touch the ground again. In the morning (well, later in the morning because it's invariably 3 o'clock by the time the mission is complete and Dumbledore has released them), he'll think about what successful means. He'll define it in terms of the depth of the gashes acquired, the number of new bruises that blossom under his skin, the amount of amber liquid it takes for him to forget (momentarily) that Reg could well have been out there last night.
He'll assess it in terms of the Fabian Prewett-sized void in his life and there'll be a dark hour when he wonders if it's worth it, if the battles are worth the war, if there truly is a light at the end of the tunnel. If Lily's there, she'll hold his hand and rub small circles with her thumb as she whispers that it's always darkest before the dawn. If James is there (Peter rarely is these days, the war is affecting him worse than most), he'll slap him on the back, tell him not to be so stupid, tell him that they're fighting for everything they hold dear. If no one's there, Sirius will remind himself that he fought- and beat- the darkness once before, that he can do it again.
For Remus, the battle-stoked adrenaline has long since disappeared. He's not even sure that it was there the first time. He sees it, though, on the faces of his friends, even though they are no longer naive seventeen year olds, even though so much has been lost. Sirius lets out great shouts of laughter in the heat of the battle and his eyes are always alive, his long hair dancing to some unheard, deadly tune. James, blessed with the perception of a Chaser and the reflexes of a Seeker, weaves in between cloaked figures, always getting too close and always retreating just in the nick of time. Lily says little but her eyes are greener, her movements jerkier than they are at rest because her passion, her belief, her commitment spur her on. They think they have reached the pinnacle, the point where life offers up its mysteries to them and death is so abstract it ceases to exist; Remus knows life, knows death, has been on the boundary too many times as wolf and as man, and knows that they do not. He knows without doubt that his expression is one of stoic concentration when he blasts a Death Eater out of the way. His assessment the next morning does not take long; his definition of a successful mission is simple: did his calmness falter, did rationale momentarily abandon him? Did the darkness, no matter how briefly, consume him?
At 3am, though, they share a definition, an assessment. At 3am, life has Sirius in its clutches and Remus is firm and unwavering; an unstoppable force meets an immovable object as the adrenaline peaks for Sirius and, finally, courses through (burning) Remus's veins. They are transcendent, both living and both alive and both beyond words, as lips and fingers touch and ruined clothes are shed, and the only darkness that can be found is in the shadows of their clavicles, the hollows beneath their shoulder blades, the enlarged pupils that reflect their own faces back at them.
When they are tired, Sirius opens a window and the sensation of cool morning air on their cold sweat is indescribable. Remus plays Handel on his dad's record player and they say nothing because there comes a point where words are only noises humans have learned to identify and taken as their own.
And they are in the ascendency.
Come, let us march against the powers of heaven,
And set black streamers in the firmament,
To signify the slaughter of the gods.
Tamburlaine the Great, Part II: Act V, Sc. 3 - Christopher Marlowe