Most sleep at night, most cannot handle the complications of flying in the dark, sleeping reduces the chance the bird will attempt night flight that results in a crash.
The birds that do sleep are safe unless you see them in the dark with a flashlight, you can slowly walk to them and capture them without a struggle than can hurt the bird
Some go to roost (snuggle up, keep warm and get some shut-eye)
Uless of course the bird is a nocturnal predetor like an Owl, in this case it go'es hunting for its supper!
Small mamals insecte and frogs etc would be on the menue
Birds that migrate prefer to do this during the hours of darkness using the stars as pointers. At anytime from September onwards flocks of Redwings come over from Scandinavia, perhaps accompanied by Redwings, so it's quite likely to either see them or at least know they're about by their contact call.