So far in The Legend of Zelda: Skyward Sword the only time of day I can visit the surface is the day. Can I visit at night, and how?
Sadly, you can't.
Unlike most recent or old Zelda games, Skyward Sword doesn't have a traditional day/night cycle. The only way to switch between day and night are the beds in Skyloft, the bed in the Lumpy Pumpkin, and the bed in Beedle's Air Shop... and Loftwings don't fly at night.
(Don't be like me and try to dive off the side of the Lumpy Pumpkin or Skyloft at night!)
No you can't get to the surface at night. To be able to get there, you need to be flying on your Skywing and they cannot be flown at night (by you). You'll need to do it in the day.