Eye-pupil: Smaller in sunny bright day.
Eye-pupil: Large in dark conditions.
In the dark
Your eye-pupils are large in dark conditions to let more light in your eyes, and smaller on a bright sunny day so this can not blind-dazzle you. This is called dark adaptation.
Your eyes adapt to depend, whatever the lighting conditions are – let taking for example;
A room at night with lights on – it all looks fine because your eyes have set themselves to work out whatever light in this room.
Now turn the lights off, the first thing you will notice this room appears almost all black for a short time.
Because your eyes have gone into dark-adapting smaller, that your eyes were lacked of light, then your eye-pupils grow bigger to let more light in.
From ten minutes
For the night sky – actually takes around an hour, but a good balance is complete within ten minutes or more then you can start to see the faint night sky (for example:- brightest comet, galaxy, globular or open cluster, and nebula that you could see by naked-eye), make sure you can not see any bright lights while you are observing the stargazing.