Canaries follow the natural light circle. They wake up with the sun light and when the night comes, they go to sleep.
But the pet canaries live in houses, where we turn on the lights and this light is not ok for a canary. They need to rest following the natural light, so cover them with a textil, that makes their cage dark enough.
If they stay awake for a longer time than their normal time or you keep on waking them up when you are entering in the room you keep them, this can have an impact in the health of your canary bird.
Covering the cage lets them sleep the hours, that they would sleep also in the nature and lets them rest.