Night guard duty is the worst.
When you first move into an area, it's terrifying. Every sound is the enemy, every movement is a surprise attack, every shadow the enemy in some terrifying form. The dawn can't come soon enough, and when the sun finally starts to break over the horizon, a wave of joyous relief envelops you. You've lived to see a new day, and as you inhale one last cigarette before you fall into your bunk...a habit you didn't pick up until you came over here...a sense of accomplishment and purpose hits you, and you're on top of the world.