Things are different at night. Sounds that you hear in the deep of night bother you in a way they wouldn't if you heard them at noon. Things can feel more eerie. Dark provides cover for many strange activities.
Jesus points out that they could have arrested him any time they wanted. It's not like he was hiding from them.
But they waited until night, in the garden, where hopefully less people were watching.
Were they ashamed? Were they afraid? We don't know. But they didn't want people to know. This was their hour -- the power of darkness.
It's a good time to be afraid. And yet... Jesus is there, and so there is still hope. Even in the power of darkness -- there is still hope.
So do not give up when the darkness is closing in. No matter how dire the odds may seem, because of the power of the Gospel, we still have hope.