I've noticed that different firmware changes the light output on the Glo quite a bit. My usual setting is at 28% and it's perfect in daylight, even sunlight (it just makes the page appear whiter), and it's perfect to read by in a totally black room for me, it's not too bright. When I was trying other firmware versions, I had to set the light to 72% for one firmware to get the same light output. Another firmware I had to lower it down to 12% for the same light output.
Like I said, Kobo's firmware is my biggest complaint, but if you find a version you're happy with and stick with it, the Glo is absolutely a fantastic reader.
Edit: There's also a hack that lets you reverse the white/black on the screen for a nightmode for those stuck with a light that's too bright. I haven't tried it, haven't ever felt the need for it for myself, but others love the option. You can read about the hack and download it
here.