Depends on your bird, although tiels are prone to night frights, which can be really bad if they get a bleeding blood feather in the middle of the night and you don't notice. You'll mostly have a dead bird by morning if that happens.
For my bird I found out that he sleeps much better at night with a cage cover. However mke sure whatever cover you use, leave either a night light inside the cover or leave a crack on the cover so it's not completely dark.
I suggest at least cover the cage at night/your bird's sleep time. As this seems to encourage a calm state to most tiels I have seen/known.