Fear leads to anger...
He fears the darkness.
During the daylit hours of waking, he can occupy his mind with the million-and-one details of his command, the little things that keep the Imperial fleet coordinated and effective. During the daytime, he throws himself into his work with a passion, earning the respect -and fear- of those who serve beneath him, especially among those who do not take their jobs seriously. During the daylight, he is strong, in control, safe. The embodiment of a warrior, he terrifies those who cross him and appears to be beyond the reach of the fear he inspires.
In the night-time's darkness, this is no longer true.
Many times, he takes reports or paperwork with him when he is supposedly resting, pushing himself to the brink of exhaustion and beyond. Many nights, he is unable to rest, unable to close his eyes for fear of falling asleep and dreaming dreams of terror. As horrible as his dreams and visions can be, these are not what he fears most about the nighttime.
What he fears most are the memories.
His memory holds greater terror than any fleeting dream. Those comprise his greatest fear; not death, which would be relief, nor physical pain, which he has endured many a time before, can bring such terror as something so simple as a memory. One may shake off a dream, dismiss a vision, but memory cannot so easily be banished.
For at night, if his mind is allowed to wander, the memories return to haunt him, leaving him drained and aching deep within. The darkness shows him his past, his failures, his mistakes, his losses. Flashes of fire and blue blade catch him unguarded at times like this, the darkness reminding him of the innocent blood staining his hands in the wake of his fall. Remembering brings pain and sorrow, enfolding him in the bleak depths of despair. He tried to avoid the night, tries desperately to keep the darkness and agony and hopelessness at bay. Sometimes, he even succeeds.
But not always.
It is painfully ironic that a Sith Lord should fear the night so greatly. After all, night-black side of the Force is what gives him his power now, sustains his fragile life. The dark side is much with him now, a fact that cannot be denied and seems irreversible.
And yet, the descent into this darkness is what has spawned these memories that relentlessly torture him with images of loss and pain. The darkness holds him in bondage, in chains of his own forging. Dark pain and darker despair bind him more tightly with each sleepless, anguishing night. The night mirrors the darkness his heart, the emptiness that drives him to shamed tears when he remembers what he used to have. Indeed, the darkness of night is torment beyond comprehension, far worse than even the torturing flames of Mustafar.
Rightly, Darth Vader fears the darkness.